Something that really helps me it to not have a fixed idea of what I want to practise. Often I'll have an idea that I want to do 100m zenskate 100m swim intervals or whatever and find that it's probably not gonna happen in the space. So I just give up watching the clock and just try and drill when appropriate. Be flexible.
Another good idea is to always bring fins to your practise. This will help speed you up on the slower drills.