1. What is your coaching philosophy?
Are you offensive minded or defensive minded? Do you like to run or play half court? What are the things that are most important to you? Your drills should reflect your philosophy.
The best drills come from the big picture. After deciding what offenses and defenses you are going play, take pieces of your system and make drills out of them.
Offensively, if there is an aspect of your offense that uses "down screens," do a lot of down screen drills. If there are "pick and rolls," do pick and roll drills. You can take the first 2 cuts out of your offense and make them a drill. Isolate each of the scoring options and make them drills.
Defensively, if you like to trap, you can run a drill with 3 defense and 2 offense and simulate a trapping situation. If you want to double the post, construct a drill where the ball goes into the post and you go get it. If you want to drive the ball to the sideline, use a drill that involves playing the ball on the dribble and force it to the sideline.
On the whole, each drill should reflect your coaching philosophy and should mimic pieces of your actual offense and defense.
2. What are your player's strengths and weaknesses?
Drill work should be designed to make your players better in addition to improving your team. That might mean different players doing different drills.
Decide what roles your players will have inside of the team and evaluate where the players need to improve to better fill their roles. If you have players you want to shoot from the wings, those players should practice those shots. If you have players you would like to play in the lane, that is what they should practice. It serves no purpose to have players that will never shoot from the wings taking time to practice from there. Players should practice the things they will do in the game.
3. How much time do you have?
When having a team practice, time is the single most important thing you must manage. In season, team development must be the priority. It is difficult to decide how much time to spend on team aspects and how much to spend on individual improvement. The shorter the practice, the less individual work you can do. The more time, the more individual work can be done.
Combination drills are a coach's best friend during the season. A combination drill is a drill that practices an individual skill inside of a segment of your system. That allows you to get some individual work in while improving your team aspects at the same time. For example, you can do a shooting drill that involves multiple passes and a screen down with multiple repetitions. This allows you to get shooting repetitions for individual improvement while working on offense's timing and your patterns.
Trying to figure out which drills to run for your team is one of the challenges of coaching. There is always so much to do and precious little time to do it in. If I were left alone, my practices would have been 12 hours long due to the fact that I would try to run every drill I know every day. Keeping these 3 things in mind will has helped me as put together a practice plan. I have been able to eliminate the drills that are not relevant to my team or take too much time while enabling me to vary the drills to increase effectiveness and fight boredom.