When five of us went to Ireland for Spring Break of my senior year in college, there was a lot of debt to keep track of. Rental cars, lodging, and meals really added up; I'd say a couple hundred dollars of expenses to keep track of each day. Splitting each little transaction five ways was out of the question; it would have been a terrible amount of effort. Now, if it were just two people, it probably would have been pretty simple to keep things fair. But, with five people, you're practically guaranteed that the honor system is going to be very nice to at least one person and really shaft at least one person. I was determined not to let this happen. So, I tracked every little thing each person paid for in a little book, whether it was a ridiculously overpriced (and not very tasty) Irish Coke for one person at the St. Patrick's Day parade, or the whole gang's stay at the bed and breakfast. Then, it didn't matter at all if one person was mooching hardcore and one person was paying for everything.
After the trip, I typed all of the numbers into Excel. Then, with just a totals row, I had a number that quantified each person's generosity (positive number) or mooch factor (negative number) in an exact dollar amount. These could then be paired up, and with just like four or five transactions, the dozens and dozens of debts could all be rationalized and paid off, and everything was fair.
I was very pleased with how it all worked out in the end. Both of the extremes in my example—the quiet, generous person and the major mooch—tend to annoy me. Now, none of the people I went with would have intentionally been the one to pay a hundred bucks less than everyone else, but it seemed silly to me to have someone who got gypped out of a hundred bucks when it was completely unnecessary and extremely easy to prevent.
I imagine that at least one of the other four people thought I was needlessly obsessing over money a little too much, but nobody said anything, and I didn't see it that way. I don't like being indebted to anyone, even if they never expect me to pay them back. Besides being fair and easy to keep track of, the system brought me peace of mind; I could be sure that I wasn't cheating my friends, without just dumping cash on the problem.