Partner, partner, partner.
I find it almost impossible to get back on track once you're off. Having a partner that won't allow you to say "Eh, I'm not gonna work out today" really helps.
For me, summer is the easiest. I'm not all that busy and I go out to the pool every now and then so that's motivation in and of itself.
It's almost impossible for me to stay on schedule in the fall because football gets so crazy busy work-wise.