It kind of depends on where you are in your officiating career. If you're just starting out, then the thing to work on is mechanics and positioning. Are your signals correct? Is your arm straight up for a foul or violation? Do you count properly and in the proper situations? In the Trail, do you step to the basket on a shot attempt? In the Lead, do you close down to the lane when the ball is on the opposite side of the court? Do you stay within your primary area of responsibility?
If you are already experienced and looking to move to varsity, then a couple other things are important. Play-calling and game management. You have to get plays right. That sounds obvious, but it takes a long time to develop. You have to know the rules and you have to exercise good judgment. You have to get good angles and get the play right. If you get the play right, assignors don't care about your mechanics anymore.
Secondly, you have to be able to manage a game. There's a lot of stuff that goes in this category. Manage the coaches and the benches. Manage the table when things get screwed up (or hopefully, before things get screwed up. Manage the players by talking to them when you can, and taking care of business when they don't listen. Manage your crew in a tight ballgame to make sure everybody's aware of the situation and on the same page. This also takes a long time to develop.
I just said what the other guys already said; I just used a lot more words.
Hope it helped a little.