How can I put myself in the best position to get a postdoc in a different field? Im a graduate student in my 3rd year studying theoretical plasma physics. I'd like to stay in academia but I think I would be better in an area of experimental physics where data analysis plays a more important role (such as High Energy Experiment). I wanted to know wether or not it was possible to get a postdoc in another field than the one I write my thesis in. I'd like to hear from anyone who has done this. 

If this is a possibility (a realistic one), what can I do to prepare myself in the remaining 3 years of graduate school? Thanks!
I'm in the same boat as you, but then for my thesis I added a few elements of the thing that I want to do later in my career, so I can use it as background. If you can incorporate something related to experimental physics in your plasma physics research, that'd help. Otherwise, you may join some research lab that works in the field of exp. physics, that'd look good on your resume.