The secret is loose face powder.My son, below, has been a tiger for his third (non-consecutive) year now and this tip has yet to fail us. (in fact, my daughter has has some white swirls on her face since Saturday that I had forgotten about due to her hair covering them up. Yikes!) Here's how it's done.
You take your loose powder and shake it on to a non-fluffy powder puff in excess. You press that powder on to the make-up firmly without smearing. Get it on every whisker, patch, scar, whatever. (unless you're making a ghost or vampire, you're going to want the powder to be skin colored)Next, you'll need a powder brush. You'll want one dedicated to this because the last thing you want is remenants of grease paint getting onto your own skin later because you used your expensive brush set. Just dust the powder off with the brush and you're done. The make-up will stay all through the school day and trick or treating. Even if you're just doing a little puppy dog nose on your baby, you'll want something to set that black eye-liner of yours or he just won't look complete for pictures. :)