In my family, we draw names. If you draw the name of a single person, you buy a nice gift for that person. If you draw the name of a couple, you buy for the children of that couple, if they have kids, or the couple themselves if they don't. There's a $50 limit for adults, either singles and couples, and if you are buying for the kids, I think it is $25 per kid--so either one really nice group gift for all the kids, or smaller, individual ones--giver's choice.
Before we did that, I had a dollar amount per person that I spent. I'd either get one gift for a couple if I knew something they wanted, or separate gifts if that worked better for me.
I have to say, as a single person, that for the most part, the couples in my family are extremely generous with their gifts--I have my suspicions that the $50 limit has been broken more than once. But then again, I may have exceeded the $25 per kid limit myself, when I found the perfect gift. We're good at creating rules, but not necessarily following them.