How can I gracefully exit my current post-doc position for a new one? I'm a post-doctoral research fellow and I've just been offered another post-doc position at another university. I still have a year and a half left on my original contract, but it's not going as well as I'd hoped and ultimately is not going to lead me to a place I want to be in my career. The new position is at a better lab with many more PhD students to work with, somewhat less research freedom but more resources and support, and also genuine guidance to prepare for life as a professor. As an added bonus, the new position is in the same city that my partner lives in, and we will be able to live together. 

The problem is, for the sake of professional etiquette, I can't tell my current advisor that this position was offered to me, as my new advisor doesn't want to be seen as poaching a post-doc from another university. I don't want to burn any bridges or cause ill-will at my current position. I'm at a bit of a loss now, trying to figure out how to approach a conversation with my current advisor to tell him I'm leaving, and I would really appreciate some advice!
I changed from one unfinished postdoc to start a new one.  In your list of reasons, there is one that easily trumps all others:


  the new position is in the same city that my partner lives in, and we will be able to live together.


Tell your current advisor that you are moving to another postdoc so you can live with your partner.  Any remotely humanoid advisor will understand and accept that.  The reason for my move was exactly this.

All your other reasons are less important, and may be interpreted as implicit criticism of your current place.  Since you have the two body problem, you can move without mentioning any other reason.