If you're going to ride fixed, then "don't stop pedaling" should be your mantra for the first few rides. after that, you'll get used to it, and actually (hopefully) enjoy the control that fixed gear gives you in foul weather.
For single speeding, remember that momentum is your friend, so every time you hit the brakes on roller descents, you rob yourself of the momentum that can help you made it up the next hill.
Both are great fun.
Good luck...