I find it pretty easy to get into the heads of my sympathetic characters, who are generally unlike me. (I did start out by assigning different of my own traits to different characters, though.) What I have trouble doing is getting into the heads of the unsympathetic characters, generally my characters' antagonists, in order to make them remotely believable. I know people are greedy, or vain, or bigoted ... but it's really hard to get into that headspace!