We always just divide the check equally.
We got burned big time though last August in Las Vegas. We went out with another couple to a restaurant called Kokomos at the Mirage. DH and I split a salad (they were HUGE!), then each had an entree about $27 each, we each had one drink about $7 each and that was it. The other couple each had their own salad, the guy got a lobster bisque as well, then they each had the $60 filet, 4 martinis at $9 each, then they each had a dessert which was about $8 each. When the bill came, the other guy picked it up and said we each owe about $150 plus tip (or something like that). I was truly shocked that he expected to just split the bill after all they ate. We paid half, but we were not happy. Needless to say, we won't be going out with them again.