#1 Don’t. It’s better not to even get involved. They always want something and they never take “no” for an answer. No one can be as persistent--not to mention annoying--as the not-so-dearly departed.
#2 See #1. What they want usually involves being followed, getting shot at and dealing with people who would definitely not get Mom’s seal of approval.
#3 The dead are incorporeal. They can’t do much of anything for themselves. Except maybe make your life way too complicated. Remember the power of a bargaining chip. They want something? Make sure you get something in return.
#4 It doesn’t hurt to have a couple good guys in your corner, even if you may not be ready to tell them about the whole I’m-talking-to-the-dead part of the equation. A sexy cop, maybe? Or an absent-minded but adorable research scientist? Can’t decide? How about both!