If at all possible, try to get a reading that you're confident in BEFORE you get into the back of the ambulance. Definitely make sure you get a good reading before the truck starts moving because if you're having trouble now, there's no way you'll be able to hear or feel it once you start moving.
Owning an expensive stethoscope also helps more than people would like to admit. Beyond that, practice and don't get discouraged. And don't ever make up numbers - sometimes it is not even physically possible to get a pressure this way due to peripheral artery disease or whatever. Feels bad man to not be able to tell the hospital what the pressure was, but you can use other proxies for perfusion (skin color, capillary refill, mental status, etc).
Do your best to get a good reading before leaving the scene. There are lots of tips in the other thread, but remember that if all you can get is a palpated BP, that's a valid result. Of you can't get even that (happens sometimes) use your proxies and go with it.
The best way to learn this above the technical aspects ( and the way I learned it) is to customize yourself to the environment. Go out next to a freeway (or other noisy location) and use the manual cuff. (taking your own BP works best because you'll be familiar with exactly where the numbers will tend to be. ) This is really one of those things where acclimation to a noisy environment is everything.