Originally Posted by
az2008
I've watched a mechanic do this. He snugs one side. Then clamps his hand onto the seatstay and tire, centering the tire and holding it firm using the seatstay as a leverage/clamping point. He snugs the opposite side, going back to tighten the first side, then tighten the opposite side. It doesn't look too bad.
When I have the bike upside-down I'll put my thumbs on the lower chainstays and push the tips against the rim. I can judge how straight the wheel is by where the chainstay is in relation to my thumbs, making it a snap to align.