I think when I'm writing characters that aren't anything like me it's a little easier, because I just make sure they don't act like me ;) but when I'm writing characters that are similar (but not exactly like me) I have to think "would he/she really do that?" because the temptation is to just insert my personality. I think the easiest thing to do is base character personalities off people you know, because then you have a definite source to work from and there's no temptation to make them too much like yourself.