Play an hour or two of it. Not enough to spoil the whole game for myself, but enough to give informed feedback.
Write up an extensive list of (hopefully all minor) complaints. Any bugs I encounter are going on there, obviously, but my real concern is larger stuff like usability, feel, grammar and spelling, game flow, exploits, etc. Bugs are problems, but they can be patched. Stuff that's there by design but doesn't work the way it should is a far more pernicious problem. I'll leave the dedicated bug-hunting to those more anal-retentive than I.

Wait until the game is finished to play the rest of it. I want to be surprised!
I do hope some people are planning on being genuine beta testers, though. I'd hate to see the first couple of hours receive all the polish and then watch the game turn into a bugfest after that. I expect there will be bugs, of course - games with this level of reactivity always have them. But we need to make a concerted effort to find as many as we can before the game is released into the wild - like it or not, review scores and initial impressions matter, and negative word of mouth will not do the game any favors.
I know the sales target is low, which is great, but I want Wasteland 2 to do much higher numbers than that. 500K, at the least. It probably won't crack a million - not until at least a year down the road, at any rate. But I think it can top the Steam charts for at least a couple of weeks and be a consistent earner if it gets enough positive press, and a big part of that is making sure it's as polished as it can be.