I don't exactly give them orders. Actually, I ask them to do themselves a favor. Usually, it goes like this:
"Do you think that at this time tomorrow, you'll be upset over this? Probably not. It's more likely that you'll have forgotten it before you go to bed tonight, so it's obviously not something that should dominate your day. Now, since we both know that you are going to move on from this issue, do yourself a favor and just do it now. Get over it because you're going do it anyway."

Sometimes I get a reaction that's positive, but a few times, they consider it, realize that I'm right, but then start their response with "Yeah, but..." I guess we sometimes are happier when we are unhappy.