I set goals that involve outside influence. Such as signing up for races and competitions. I don't expect to do well, it just helps with motivation for me (If I don't do x this week I'll be behind for y competition).
Additionally, I give myself wiggle room and schedule by week, not day. I make sure I have days built in that I don't have anything, so if I'm really not feeling it one day I plan to workout, I have another day that week to make it up. It helps me put less pressure on myself each day and I have less of an issue getting all my workouts in within a week because I know it's flexible.
So basically, I keep the pressure to exercise long term (competitions) and let the short term be very flexible.