Q:

How to end a game session smoothly

After my first (failed) attempt at playing tabletop RPG two years ago, I'm going to give it another try. Still as a GM.
One thing I didn't do quite well last time was ending the game. I didn't know what to do to properly end the adventure.

"Well, congrats, you killed the dragon. Hope you liked it. Here is your XP. See you next time."

Browsing the site, I found this question : How to make my session Faster.
I'm not interested in a cliff-hanger sentence so that we jump in the action next time. The pace is not an issue.
I'm more interested in something like

"Ok, you killed the mega boss/you solved the mystery/you returned the stolen item. You go back home and enjoy your rest"

How do I end a game session ?
A great answer would provide techniques to :

let the players know that the game is ending
smoothly distribute rewards (and not just "oh you killed him, +2000 xp") 
introduce an "after-game" mini talk session to discuss what they liked/dislike

A:

Managing the end of the game
I'm very visual so I tend to think in terms of movies or tv shows.  So if it's not a continuing game where cliffhangers make sense, then I wrap it up and maybe ask a few questions to give the players some input.

"You come home and the elders greet you and feast you! You're the
  heroes and a celebration is held..."
"Did you want to take a moment and play out what you say to your
  husband now that you got back safe?"
"You got a bunch of treasure, were you going to open a school or build
  a temple or something?"

It might be a sentence or two, or a short 1-2 minute scene and mentally I imagine it's the montage you get while the credits roll.
Managing the end of play
I also like to take a little time and geek out over the cool parts.  "I can't believe you just JUMPED into it's mouth! That was awesome!" or describe some of the stuff I had kept hidden during the adventure "Yeah, the dinosaur does 20 points of damage! I'm so glad it didn't hit any of you guys!"
I ask what the players liked, and sometimes hint at what could happen if we played more.  "But now you own a temple! That means we could totally play out the struggle between the Reformers and the True Light Disciples next time!!!"  "Your ninja can literally jump through shadows now! The fights are going to go way different now that you can do that."
Post Game Info
Sometimes I like to write up the experience and post it online.  Tag or link your friends who played and let them comment or talk about how it was for them as well.  It's fun for your friends especially after they've had time to think about it, and also for other gamer/geeks who want to hear about how your game went.

A:

Choose your Moment
A session should usually end at the end of a scene (unless you're doing a cliffhanger ending).  Let the final scene reach its natural conclusion then tell your players "That's all for tonight.  Thank you for playing."  You don't need to use those exact words, but the idea is to have a firm statement that the action is over, and a positive acknowledgment of the players' involvement.
Where does the scene end?
As a new RPer, this might not be clear right away (if it is, just skip ahead to "More precise timing").  A scene is a chunk of your narrative, one of the "sub-stories" in your game session.  A scene will be a sequence of actions that are tightly connected.
Think about scenes in movies.  It's mostly the same.  A scene change is often when characters move to a new location, or start doing a different activity.  Here's an example of scenes a fantasy adventure session might have:

Figure out how to open the secret cave-dungeon door
Travel through the first few corridors of the dungeon, avoiding the
hazardous chasm
Run past the cave troll before it grabs you
Find & Kill the dragon; it was evil

In this example, most of the scene changes happen because the characters are going to a new place, but the important part is that the characters are doing something distinct in each scene.  Scene 2 could have spanned multiple rooms and tunnels, but the action is basically the same throughout.
More precise timing
Once the major action in the final scene ends, when do you say "That's all"?  Immediately after you narrate the dragon's death (or whatever the important action is) is one place you can end things.  "...its breathing stops and the room grows silent.  That's all for now, thank you for playing."  
This is slightly abrupt.  That's a good thing if you want to underscore a major event that just happened.  The jarring end will leave players with a sense that something big just happened.  The aftermath of the event matters, but we don't get to see it yet.  Players will leave the game with the final moment etched in their minds.
Another option is to let characters "clean up" at the end.  Usually, characters want to do things like exchange a little dialog, search the bodies, or check each other for injuries after some major dramatic sequence has ended.  For a less jarring ending, let your players do this.  
Listen carefully to their actions to pick that right moment to end things.  They will probably have a few things they want to say right away ("My character claims a dragon tooth trophy", "Sally, are you hurt?!").  Let them say it, and resolve the immediate actions.  Then, declare the end of the session.  Be careful not to let the players keep going too long here.  
Rewards & Post-Game
Immediately after you declare the end of the session, start the post-game rewards & wrap-up.  In most systems* the GM is going to do all the talking for the rewards portion, so go straight into the post-game Q&A when you're ready.

That's all for tonight.  Thank you for playing.
You each get 5xp for slaying the dragon.  Bob, you get 1 bonus xp for
  excellent roleplaying.  Sara, you get 1 bonus for landing the deciding
  blow.
So, what parts where the most fun that session? [GM-led discussion]
Are there parts you would like to have done differently, or didn't enjoy? [GM-led discussion]

*Some games, like Mouse Guard, have the entire group discuss and decide how to assign rewards.  Post-game works basically the same there, except the GM is acting as a discussion facilitator rather than monologuing.

A:

Ending the Game
In general, I don't end a session immediately after a boss fight (or any fight, really) unless I'm pressed for time. Typically I allow (it's more of an unspoken requirement really) my players to return to town/home base/wilderness camp/whatever-the-next-goal-in-the-game-is and I attempt to foster a small social encounter between the PCs. Sometimes this is going back to the person who gave them the quest. Other times it's planning what happens next. Other times it's simply them congratulating each other on a fight well fought (or being frustrated at each other for a lost fight). It really can be anything.
While my players didn't immediately equate this to the end of the session, and I still had to explicitly awkwardly end it ("Okay guys well I'm all out of stuff haha awkward laugh") eventually, (like 2 or 3 sessions) they began to realize what I was doing and began to recognize the end of the session. Once they recognized it I stopped having to say anything, they simply knew and were content to quit when they felt that their characters had said enough in the social encounter. If you're smarter than I was at the time you can even tell them that you are going to be attempting to end the sessions in that manner which means that you don't need to wait for them to catch on.
I do want to note that don't think many players put much stock in the method by which the session ends. Unless your players make it a big deal, it may not be. However, when I began DMing I too felt the awkwardness of ending a session by just saying "we're done" and looked for ways out of it. Then I realized that my players really couldn't care less if I did that (though they enjoy the method I spelled out above).
Smoothly Distribute Rewards
Assuming, like you mentioned, that we're talking about "meta-rewards" (I just made that term up) rather than in game rewards (loot!) there may not be a good, smooth way to do this. What I mean by "meta-reward" is an award for the players, as opposed to the characters. The reason that I say there may not be a good way to hand them out is because the characters in your game have no concept of experience, that is a reward purely for the player (so that they can develop their character further).
To avoid this in my games, I simply grant level ups to the party at times I see fit. If you're running published adventures then it is likely that the adventures may even say "The PCs should be level X by the time they get here." If that's the case, just make sure they're level X at that point. It does make the DM's job easier.  Additionally, I don't do this while the characters are out on the table (again, because it's a meta-reward). I either tell the players after the session is over or before the next session via email/text/phone call/homing pidgeon. If you still want to deal with experience you can easily calculate the experience that the characters would have received and give this out to the players directly. 
Lastly, I want to foot stomp underscore my closing paragraph of the first point. In general, I don't think the players really care how they find out that they are getting to level up. Nine times out of ten they'll simply be happy to get that new feat/skill/whatever-your-system-allows.
After Game Mini-Talk
This is my favorite part about DMing, honestly. It's when the players tell you what they liked (which makes me feel good about DMing) and what they didn't (which I appreciate as feedback and attempt to fix). My group (once again, time permitting) usually eats dinner together after the session is over which is the perfect time for this mini-talk. Sometimes we cook (long meals like pot roast), other times we order pizza. Sometimes we even let the game dwindle on into dinner by conducting purely social encounters over dinner. 
You and your players' schedules may not allow for dinner after but I am certain that if you request feedback from your players, you will receive it. There are times when we haven't had time to eat and I've simply had to shoot out an email to everyone asking them for their feedback. That is okay too. It shouldn't be hard to get your players to talk about what they did and did not like. 

