Originally Posted by gugie

If the frame were going to be repainted anyway, fixing the dent is a no-brainer. If not, it's an expensive cosmetic fix. If you're handy with an aerosol spray can, you could do this as a DIY project.

1. Roll out the dent as best possible - see the link Chuckk posted above. As noted before, you can make frame blocks out of hardwood and a forstner bit. It won't be quite as good as aluminum frame blocks, but it's a zero cost option for people with a few woodworking tools.
2. Sand inside the dent and around it to bare metal.
3. Fill with JB weld, or some other epoxy based filler. Let it harden per instructions (typically a day), carefully file then sand down flush.
4. Generously sand around the damage
5. Primer
6. Paint

To match the paint, automative shops can "paint chip match" to closest color (out of thousands) and make up a spray can for you. Last time I did this it was around $20. If you want an exact match, leave the frame with them and expect to be charged 3-4X as much. Properly done you can get it to 10 foot distance match or better.

I fill dents when the frame is getting other work - repairs, 650b conversions, etc. If you're powder coating, I use silver or brass to fill the dent on the recommendation from Groody Brothers. They've found that sometimes epoxy based filler outgasses when you're heating up the frame to powder coat temperatures.