The lion's share of preparation from an assistant coach comes in practice and before game day. A good assistant coach prepares his players for the opponent and their personnel as well as preparing their coach for tactical decisions. A good coach, whether head coach or assistant, is a coach who does not need to teach anything to his team on game day.
Head Coaches all differ on what their assistants should be doing on the bench during a game. Some like to be standing the entire game, rarely talking to their assistants unless it is a timeout. Others sit on the front of the bench listening to their top assistant; others flank themselves with an assistant on each side so they are constantly surrounded by ideas. Even receiving ideas can vary from coach to coach. Some want only to ask you questions about what to do, bouncing their ideas off you for approval. Others want you as the assistant to give tons of suggestions (whether you would support implementing them or not) so they can decide what to do with a variety of ideas. As an assistant, you must get a read for your head coach and how to deal with him that best suits the program and his coaching style.
But assistants need to do more than simply take in the game, watch and take or make suggestions to the coach. There are a few things that every assistant should do or have with them during a game:
Each assistant should have a separate role on game day, watching for different aspects of the game and being in charge of scheming them. Most high school and even college teams are limited in how many bodies they can have on the bench. Piling too much on your only assistant during a game has a negative effect, both on the preparedness of the players and the flow of ideas between coaches.
Here's the way I would prioritize information by assistant:
1. Foul Chart and Timeout Planner
The lead assistant is your most trusted confidant and who will help you make the best decisions throughout the game. You should trust that they have a similar feel for the game as you do; as the head coach you don't need charting or writing things town in order to remember what is taking place in the game.
The essential information is found in foul chart and timeout planner. Have a list of each player on both teams and keep track of how many fouls they have. Also keep track of both team's timeouts, and during timeouts when the head coach is speaking to the team, check in with the official scorer to make sure your information is accurate.
2. Offensive Charting
This may just be my bias as an offensive-minded coach coming through, but I believe seeing what plays work and do not work are vital to the flow of the game. There are only so many defenses you can call in a game; an offensive playbook is usually deeper than a defensive one. Since an opponent will usually change their defense less frequently as well, you should know what plays are effective and which ones they are taking away.
In a separate article I will give an example of what I believe to be an effective chart to have on the bench. Whether it is an offensive shot chart, a list of plays your team runs or simply a breakdown of who is scoring or assisting on your baskets, this information will help you effectively call the right plays down the stretch.
3. Hustle Charting
The stats that aren't there in stat sheet and won't be kept by the official scorer. As a coaching staff you should strive to reward players who do the little things that help your team but often go unnoticed. Deflecting passes, diving on the floor, taking a charge, those plays matter tremendously. But so does taking note of when a player is not doing the little things: getting backdoored, not blocking out successfully. This gives you tangible evidence as to who is putting forth acceptable effort and who is willing to gain you and your team extra possessions. Again, I will give an example of what I believe to be an effective method for charting hustle in a separate post.
A team with more than three assistants can get creative with what the rest of the staff does. Shot charting individual players, charting individual defensive possessions, doing +/- stats for lineup analysis, many more possibilities. Most high school and even some college programs only have three or fewer assistants on the bench at a time. There's no shortage of information to be had; gather as much as you can without overwhelming one individual assistant. They need to watch the flow of the game, not have their head in a pad of paper and a clipboard the entire game.
Head Boys Basketball Coach, Boys' Latin School (MD).