The trick I've seen best results with is to tell yourself, "I'll just do a short workout. I won't do A, B, C and D like I'm scheduled to. I'll just do A and B today, because I'm tired."
Once I'm thoroughly warmed up and have some fun with A and B, I usually am in the right mindset to finish the whole workout, including C and D. If not, I have to make the decision whether to push through or cut and run. That decision is best left to a coach or workout partner, but if you have to make the call yourself, then that's how the cookie crumbles.
This trick doesn't work in some instances, like during the last weeks a very difficult novice barbell progression. In those cases, you just need to harden up and do it.