I'm afraid to say it is just practice practice practice!
My gymnasts aren't allowed up on the beam until they can do this on a floor line consistently - this has 2 benefits...
1) They don't get on the beam until they are 100% ready physically to do the skill
2) They don't want to spend too much time on the floor, so they work so much harder trying to get it right and be the first to be allowed on the beam!
However, having said that, I believe there are exceptions to every rule. If a gymnast finds a way of performing this with a different hand placement which works well for them, then I consider letting them use it.
Maybe you could, as the previous poster suggested, get your coach to spot you on the floor, and maybe just give you a lift (a bit of extra height and therefore time) to get your hands in the correct position, or practice bwo with the same hand position.
I have also found occasionally that gymnasts automatically get their hands in the right position when the HAVE to! (ie, it doesn't really matter on floor as they aren't going to hit their head on the beam, but when they are on the beam their survival instincts kick in and they get their hands in the correct position every time!)
Hope this helps and good luck!