If I mess it's generally in the morning - I tend to be very regular, and if wearing a diaper, at some time not too long after getting up it will happen almost without control - a wonderful, babyish feeling. I usually head for the shower after 15-30 minutes at most. I don't find cleanup too bad - thanks to a shower head on a fairly long extension it's easy to direct the water where it's needed.
For me the motivation is the helpless, babyish feeling of the heavy, messy diaper. I don't like to sit in it or play around, though I will sometimes gently squish my diaper with my hand (from the outside!).