Libv will probably have more suggestions, but the most important things IMO are providing good, useable feedback and trying to minimize the amount of developer time required to track down problems :
- pick up the latest driver on a regular basis and try it out
- if you find problems, try to find where the change was introduced before filing a bug or email, ie if there were 40 changes since the last time you tested go back 20 and see if the problem is there, then back or forwards 10, 5, 2 etc.. until you find the offending commit
- where possible, try to eliminate configuration issues before filing bugs or emailing the support list. Right now this isn't a big deal because the driver is relatively simple (ie just user mode, no drm etc...) but as more features are added install and config issues will become part of the challenge just like with a proprietary driver
- exercise the new features as they are introduced, but try to think and provide feedback more like a good tester than a demanding user
- if there are multiple drivers offering support for your product, treat it as an opportunity to identify the good parts and approaches from each so that eventually all the best parts can end up available in one driver; don't treat it like a friday night cage match
- try to understand and appreciate the long hours these guys are working. If you have any doubts, go look at the IRC logs.
conntest output, X logs, lspci output. sometimes a copy of videocard bios is needed.