For me (and I would imagine most) my tolerance is dictated by the previous 24 hours... What I've eaten (starches are always good) and how much water I've consumed. If I haven't eaten a lot lately I might be calling it quits for the night after 6-10 beers, but if I've been good, then I could probably polish off a case if I ever kept count.
However, once I have 2-3+ beers, when I get home for the night I'll take a vitamin C and a B complex pill, chase it with a double shot of pickle juice and put down a quart of water. As bizarre as it sounds it works. Mostly it works because of many point listed above. The water targets dehydration, but so does the pickle juice. It's been determined that pickle juice has about 15 times more good electrolytes than Gatorade which has double of poweraid.
The vitamin C helps to target some of the deficiencies that the beer creates, and the B complex helps to boost your metabolism to process said alcohol.
I may not sleep real great that night (mostly due to the B Complex), however; I can say that I almost never have a hangover by morning and the few mornings that I do I really deserved them... Generally those nights included substantial amounts of liquor (read shots) on top of a whole lot of beer.