There are so many ways people do it these days I don't think there is a hard and fast rule. DH and I haven't had a formal wedding yet but if/when we do I would pay for the dresses/make-up/hair etc. Maybe ask the bridesmaids to buy their shoes and pay for any little extras like a spray tan etc. On top of that I would give each bridesmaid a small gift of thanks. In return I would expect them to do a proper job of actually helping me to organise the whole thing!!! I hear so many stories of bridesmaids just kinda going along for the ride and being too busy to help the bride look at reception halls/book flowers/ organise the cake etc.

I also wouldn't expect them to buy me a wedding gift. If they did that would be up to them... it wouldn't be expected.