I warm up full turns by starting in a lunge and then going on releve (foot at passe), arms by ears, hold for 5 seconds (you can hold for longer if you want). The key is to keep your weight over your big toe and to move your arms from the starting position to right above your head-- no winding up necessary. A lot of girls use their arms way more than they should, and this drill helped me to learn to pull straight up and not drop one shoulder.
I don't really know what your specific problem is, but I focus on stretching tall to the ceiling (and staying on high toe), being tight in stomach and butt, and spotting the end of the beam - so I know when to come down. Mainly it is stretching tall and tight, I think.
Also, don't drop your shoulder at the end. I can't tell you how many girls I have who land the turn, then drop one arm down. You know that you're doing this if you always fall off the same side of the beam. Stretching tall throughout the turn should eliminate this, but so many gymnasts stretch tall until they think that they have finished the turn only to fall off (usually on their weak side, a lefty usually falls to the right of the beam). Keep stretched tight and tall until you have landed the turn, stepped out of it and are into your next skill.