Change into your workout clothes as soon as you get home.
I heard this from a coworker and I thought I would share. I've been trying it myself, it's uber simple, and it really works!
I usually want to throw on comfy clothes upon walking in the door from work, so why not make it workout clothes? From there, you set the tone that you are going to the gym. Maybe not for a couple hours, but you're sub-conciously telling yourself it will happen today.
Try it today and then come back and tell me if it worked for you too.