I think it's necessary to approach a double-back simultaneously from two different angles.
1) The physical angle. Learn to generate enough power, enough height, enough rotation, to do the skill successfully. The most crucial part here is the roundoff (and to a lesser extent the backhandspring); it's gotta be perfect, and it's gotta be lightning fast.
2) The psychological angle. This side of the skill is often overlooked. You need to know exactly where you're at in the rotation in order to do the skill safely, and you have to have enough confidence not to bail on the skill. The most crucial point here is the transition between the first and second flip. This is the point of no return, where you have to commit to the second flip. Second guess yourself here, and you're in big trouble.
In my opinion, the best way to train both of these is simply to do a million double backs into a pit. Not only to practice getting the height and rotation, but to practice seeing the floor. You should be able to distinctly see the floor after both the first and second flip; this eliminates any guesswork involved in landing the skill.
Double backs on a trampoline will help this as well.
As I said in another thread, it's not necessary to try to stick the skill right from the start; with tsuks and double backs, I allow and even encourage my kids to roll backward out of the landing until they are 100% comfortable with the skill.
As for timers, once you already have the skill, the best way to warm it up is to simply do a back tuck as high as you can. The goal is to kick out and spot the ground while still on the way up.