The first thing I do is check the slot for obstructions/dirt with a toothpick. You'd be surprised at the crud that can build up in old film holders. (especially if the original owners were heavy smokers... which seems to be just about all of them) There are TWO tracks... one for the film, an one for the darkslide. You might as well clean them both; if one is dirty, the other most likely is as well. Don't bend the metal guides or you are sunk. Make sure you check the slot in the loading flap as well. I clean my darkslides every few months, paying special attention to the edges. I then rub the edges with a bit of candle wax and buff them a little. If you've got a bad holder, this won't fix it, but it should be sufficient for ones that are just stiff from lack of exercise.
I'm sure others will have more experienced advice to offer, but this has kept my slides working fairly consistently.

Let us know how things work out.

Cheers,
Tom