Training for fly half is a toughie! Obviously the skills needed are excellent passing and kicking, as well as a good bit of speed for making breaks. However, by far the most important qualities aren't physical. You need to be able to look at a defence and know the right thing to do, for example if you see an overlap get the ball down the line quickly, if you see the full back out of position you'll know that you can put a kick over the line - that sort of thing. Experience is the best thing for this really, watch a lot of the top number 10's in action, look at what they do in different situations. Game time at 10 is also a must, going from playing physical, hard hitting rugby to playing a more mental game is something which takes time - be patient!