Basically, once the Preview client is considered release worthy by LL, it should _not_ be released. Instead, a little message should appear, perhaps upon entering the client, basically telling the user something like:
"A new version of the Second Life client is about to be released soon which may affect compatibility with some machines. To be sure you are not affected, it would help us if you were to try the Preview Grid.
Would you like to download the preview client now?"
Clicking yes would send your account name to the LL site which could request your account details to be transferred over to the preview grid (so people who were not able to preview before can get in for this test). It then downloads the preview client into the same folder as your SL program with (Preview) appended to it and opens it. You then log-in and you've got yourself a new person testing preview, give them a notice (if it's their first time) indicating that the preview grid is independant of the main grid so changes are ignored blah blah.
If it crashes, then opening either the preview again, or the main client will give them a message saying "The latest preview appears to have crashed on your computer, would you like to submit a crash report to help us fix it?" and it'd go to a special preview crash reporter.
As a note, this would be much easier to do with a good patching system, whereby you only download a 'patch utility' to change from one version to another. Not only HUGELY reducing bandwidth use for the downloads, but it encourages people to test if the patch is only 1mb instead of 35mb each time.
This would only be for the "Final" preview version before it is intended to be released, if bugs are found then it can be delayed since a release date should not be given (though all going well this preview indicates one week to update to allow plenty of testing).
I feel that if users are more actively (and conveniently) aware of the preview, and especially possible problems to come if testing doesn't reveal it first, then they will be more likely to give it a shot. It also means that more people can see the next release and be ready for it.
Once the preview is released, then if no update is required from the preview you downloaded, it could simply rename it and over-write your main client
Conclusion
Not only does the idea encourage wider participation in the preview, it also means that if the new version then doesn't work, you have no right to complain so lividly as you were asked nicely to help test it