There are many, many ways to score drills, especially wash drills. You can make big points, small points, points in a row, points only if certain conditions are met, minus points, x out of y attempts, etc, etc, etc. However, the two principles that should always hold are that a scoring system must motivate in the correct way and reward (or punish) undesirable behaviours or outcomes. But like everything the coach must pay attention to the what is actually being rewarded and not what he/she thinks or expects is being rewarded.
There are a couple of examples that come to mind. The first is related to wash drills. When a team wins (or loses) a point the situation is normally clear. What happens after a ‘wash’ is not always so clear. Often the winner of the first rally takes the serve, or the serve alternates. But it isn’t always clear that the team right team is rewarded. I recently started to give the serve to the team that lost the free ball. As the receiving team has an advantage, a team should not be able to benefit from losing a free ball opportunity. So if you lose a free ball, you have to serve.
The other example is the classical serve and reception drill. Under the most typical scoring system, the receivers accumulate good receptions and the servers are punished for too many errors by the giving up an extra point to the receivers. Except are they really being punished? The answer is a very emphatic NO! Giving the receivers a point for service errors is the worst possible scoring system. The servers have no incentive not to make errors because every extra point for the receivers brings the end of the drill closer. By not concentrating on the serve, servers miss practice opportunities and receivers getting extra points without earning them means they have also missed a practice opportunity. It is just a terrible scoring system. To provide the proper motivation and reward, service errors should result in the loss of a point to the receiving team. Lack of concentration is no longer rewarded. Watch the level of focus change instantly.
Motivations and rewards are an important part of the training environment. Be sure they are actually motivating and rewarding what you want them to do.