With good friends I generally just split the bill evenly, but that tends to be non-scalable.
One method I found which seems to work quite nicely is each person puts in what they think they owe, and any difference between that and the total bill is split evenly between the group. This tends to work out nearly-equitable and is very fast.
I generally offer more if I feel I've consumed more - "hey, I had four drinks and you had two, you don't have to pay for me" - but if they insist on splitting evenly I am OK with that.
Same thing if I'm the underconsumer - I don't want to show up at 2230, have a beer, and split a check that's been open since 1700, but if the differences are small then even split is just fine.
One method I found which seems to work quite nicely is each person puts in what they think they owe, and any difference between that and the total bill is split evenly between the group. This tends to work out nearly-equitable and is very fast.