Ki exercises--they're amazing little movements that are so simple, and yet so helpful. When I'm at home, and itching to train, but don't have a class until such and such a day
, I move the coffee table in my living room, and do lots and lots of ki exercises. They help so much with movements like pivoting and hip motions which can then be utilized in technique. They are also great for when you want to concentrate on weight underside. If you don't know how to do them, here's a link: http://www.bodymindandmodem.com/KiEx/KiEx.html
If you simply want to work on technique, I have spent much time doing open-handed techniques, as Colleen suggested, and it helps to get it straight in your mind, so it's easier to remember the next time you do it in class.