Consider going to the Disney park on really off times. Such as January, February and March (not during Spring Break Vacation though). These three months provide little to no Disneyland lines during the week and light lines on the weekend.
If you go on vacation during the summer months, consider avoiding Disney on the weekends. It is the worst during Saturday and Sunday in the summer at Disneyland. On those days you will be lucky to even find a spot to watch Disney fireworks.
Tuesday and Wednesday are the best days ever to be at the Disneyland park. Tuesday is a little lighter than Wednesday, but hey if you have to wait in line 20 seconds it would be okay, right? These two days are lighter year round and can be great for the kids who love to ride a lot of rides and packing in the fun on vacation.
Lines are bad during the vacation holidays. Halloween, Christmas and even special events can bring in thousands of people so avoid those times to avoid lond Disney lines.