I've experienced very slight motivation-enhancing effects from various stimulants. But the only thing that I've found to really work so far is falling in love. Unfortunately, the side-effects are enormous and it doesn't work under arbitrary conditions, either…

A data point: compared to the median of the population, I'm high-functioning in terms of IQ, but not in terms of the ability to get things done. The few times I've tried modafinil (due to the headaches and nausea I get from it, these have not been more than a handful), it has helped a lot with the latter.