Actually, I've never had an issue with players backpedaling on me. If they need to perform a social skill, for instance, the Politico generally just steps up and does it, so there's no problem. Same with most skills. The players know each others strengths and weaknesses and will generally just send in the one with the appropriate skill set. In fact, they usually jump at the chance to make a skill check. Nary a droid walks by without the Outlaw Tech wanting to cannibalize it for parts with a Mechanics check
If you're having this problem, however, I just wouldn't allow them to back out of an action once they've declared it. If someone specifically says "I'm going to sneak up and see if I can hear what they're saying", then I say "OK, make an opposed Stealth roll with one challenge and two difficulty". I don't give them a choice, I just ask them to make the roll. At that point, I wouldn't allow anyone to say "Oh, well, in that case, we'll have X do it because he has good stealth." They declared the action, so they do it.
I also wouldn't take the bait if someone started fishing for task difficulty. If someone asks "Can I make a Charm check to [do whatever]?" I'd say either yes or no. If they asked "What would the difficulty be?" or "Would it be more difficult to use Charm instead of Coercion?" I'd say "There's one way to find out, isn't there?"
Edited by OggDude, 09 September 2014 - 12:13 AM.