I know it can be difficult to plan weekend shops during the week, when you don't know how you're going to feel until the end of the week. When I began shopping, I would often find out that OT was available a day or two before my weekend, and I always chose that over shopping.
Sometimes, it can pay off big to work with schedulers through the week to schedule good paying shops on the weekends. As others have said, signing up with as many MSC's as you can will let you see more available shops, making it easier to plan out profitable and efficient routes.
Also, some MSC's don't allow shoppers to self-assign until they've completed x number of shops with good scores. I don't recall who off-hand, but once you prove yourself, then you become able to self assign.