Some people don't take too kindly to being tailgated, believe me. If you're drafting behind a bicycle, ask first, then get up as close as possible without touching the back tire. If by chance the bicyclist slows suddenly, you have only to straddle the tire, and if necessary, put a hand on the seatpost for a bit of braking assistance. Skaters don't have nearly the same acccelleration or decelleration on demand as do bicycles and motorcycles, so it's incumbent on the person providing the draft to speed up or slow down very gradually so as not to drop or stack up the skater. A typical scenario is for the drafter to create a gap, then overcompensate by braking too much, then taking off too fast again, and so on, leaving the draftee in a constant no-man's land.
Constant communication is the best way to keep together, and the simpler the better.
If I want my motorpacer, Greg Wong, to bump up it up or down a notch, a simple thumbs up or down suffices when he's constantly checking his side view mirrors. If the speed is just right, I'll flash him a flat hand. If you begin falling out of the draft and swimming on your own, don't be lulled into complaceny and tell yourself you'll work your way back on gradually. It won't happen. Throw everything you have into it, including the kitchen sink.