Q:

How to plot successfully in Crusader Kings 2

I'm quite familiar with basic game mechanics of CK2 & playing all add-ons up to Conclave. For context sense & realm scale: I'm a King of Eire & Wales who grew from a count in the newby-island with the earliest start in the game possible (late 8th century).
All over the Internet people are sharing expansion through murder strategies, but in my experience I find it very hard to get plot power to get anywhere close to even 100%. I want to use this approach to power growth as well, so the question is:
How to plot successfully in Crusader Kings 2? Specifically:  

how to identify targets (whose death will be of value to me) that can be  easily killed  
how to increase plot power if swaying via gifts is not enough

A:

Vyndicu's answer focuses on plot power, so I will focus on finding good targets.
When you are interested in gaining a specific title through murder, find out who is in line of inheritance of it. You can do that by hovering on the shield-icon of the title on most GUI windows. Unfortunately the tooltip does not allow you to click on any of the names, so you need to type them into the character finder (shortcut .). When you have a good understanding of the different inheritance rules, going through the family tree of the title holder is another way to find them.
Then you just need to make that person your vassal. When it is a woman, you can try to marry her to one of your vassals (you might have to make her a widow first). When it is a man, you can try to invite him to your court. Both require them to not be a ruler.
When you are looking at people among your vassals who have a good chance to gain a title through a convenient death, check out which one have claims. A claim is usually due to a short family connection to the actual title holder which means they might be close to it in line of inheritance.
Still no luck? Then just go to the Intrigue window and click on "Click to choose a plot". It will suggest you some murder targets the AI would choose if it were you. The first in the list will usually be people your character just has a very negative opinion about, but there might also be some good suggestions in the list of people whose deaths would benefit you politically.
No good target in reach? Well, then you need to make one. 

Marry someone from your court to someone who is in line of succession of an interesting title. When that inheritor is a man, make sure it's a matrilinear marriage (this usually will only be accepted when the man is rather far down in inheritance and the bride is a really high-level noble).
Wait until they have a kid
Give the kid a landed title in your realm
Have unfortunate accidents happen to everyone higher in the inheritance chain so the parent gains the title
When the parent dies (naturally or due to your help), the kid will gain the title and unless the title is on a higher level than your primary title, the kid will stay your vassal.

A:

My answer is concerned with raising plot power alone. As I am not well verse in the rule of succession myself.
If you have the Way of Life DLC, you can set your focus to intrigue.
Look for an insert king/duke/count that has a low intrigue himself or his/her spymaster intrigue is low.
It also help a tiny bit to find someone that has low diplomacy score (other npc beside you will like them less) which indirectly makes it easier to find people that dislike them enough to join you.
You can look for family member that was not happy with the current ruler (My brother got the throne?!) and invite them to become your vassal by grant a new title and press claim.
You can also bribe people to encourage them to join the plot. Up to a point although there will be people who flat out refuse to join a plot due to traits or "political reason".
Be sure to avoid invite people that has low score because that raise the chance of someone discovery your plot.
One final tip: try to invite people who are in a strong position within the court itself especially council (spymaster of your target will raise your plot power by a large amount as opposite to some minor peasant lady).

