Being someone who doesn't drink, I rarely go to any pubs (unless eating) so have never really followed what the rules are on who is supposed to buy what and when. The main occasion I do find myself in a pub is when we at work go for our lunchtime Christmas drinks (that typically span over the entire afternoon) and I have come up with one rule that has served me very well over the years and that is to make sure I buy the first round. Normally there'll be about five or six of us (out of the 20 or so in our office) who will turn up all together, so with all of them plus any people already there who are now finishing their first drinks, it means it can be quite an expensive round, but it works out in my favour later on. For some reason, the drunker my colleagues get, while they struggle to remember who bought the last round, it seems that buying the first sticks in their mind and even if it was now hours ago they will insist "nah, you've already bought recently, I'll get this one for you", meaning I'm sorted for soft drinks for the rest of the night. Not that I can stomach more than a few pints of that sugary post-mix stuff without it running straight through me, so I'll normally turn them down anyway, but still (to me) it's been an interesting phenomena to observe.