I make mine in a Char Broil electric smoker rather than a dehydrator. I start with a nice, lean piece of beef (usually London Broil but I've used flank steak or whatever else the grocery store has that looks suitably lean). The leanness is the most important part - fat and marbling is great for steaks but not for jerky. I freeze it, then let it sit on the counter for about an hour so that it's defrosted enough to slice, but frozen enough to maintain its shape / slice thin / consistent.
I use a different marinate every time, still experimenting, but usually a mixture of about 1/2 Worcestershire sauce and either soy sauce or a teriyaki sauce, with a good helping of paprika, salt, some garlic, maybe various other things depending on mood. Marinate the beef slices 12-24 hours, then put it in the smoker, typically about 4-5 hours at 150 degrees. I change up the wood chips in the smoker as well but I've tried mesquite, hickory, apple, and cherry. Mesquite and hickory have turned out the best so far.
Some batches definitely turn out better than others. Once I used almost all soy sauce and because of weather couldn't run the smoker, so it marinated for 96 hours before I cooked it. It was so salty it was practically inedible. Others have been excellent, you really have to play around with it till you find the right balance.