Supplement-wise? You could try some caffeine/coffee before you work out, though that only works for so long. But it might be a quick fix in the short term if you don't already have a high caffeine tolerance.
Interestingly, though I didn't buy it for "motivation" (more for strength/energy), Lightspeed helped BIG-time in the that area. MAJOR, non-wired mental/physical motivation and intensity booster. Pretty cool. You know how some days you just have to drag your ass to the gym, then when you get there you lie down on the bench to start benching and you just want to close your eyes and go to sleep? Not with this stuff. Pretty freakin' cool. Haven't had a lame workout since I've started using it, and that's sayin something.
I was looking for it in the stores near me but couldn't find it so far. I get it straight from their site, www.lightspeedlabs.net
You could also try the various ginsengs, but I found them to be more "calm your nerves" type supplements for the most part, not really "motivating." Except for siberian ginseng, which just made me REALLY wired, and not in a good way. That was no fun at all.