In my dark cabinet live bottles of: olive oils, vinegars. Once I open mustard, I move it into the fridge. So when I mix these ingredients together, and add mustard (salt and pepper), I feel the need to refrigerate the leftovers if I make more than I need that night. Then, the next time, I forget that the mixed dressing is there, and end up with yet another batch in the fridge. Or when I do remember, the oil inevitably has solidified and I'm not able to use that batch right away, and there I go, again, more of a mix.
Recently I read this, which appears to say it is fine to keep prepared mustard at room temp if used within a month(contrary to the directions on bottles/jars): http://homecooking.about.com/od/foodstorage/a/mustardstorage.htm
So- what do you do? I'm not talking about huge batches. If I kept it at room temp, I would probably use the rest within a few days to a week.