Same predicament every year for us also. We decided one year that we'd each buy our own present, wrap it and the giver had the surprise of not knowing what they were giving, worked quite well, except he didn't like what he'd bought for himself.
At the moment, there's a Tesco carrier bag on the dining table with a very bright yellow book inside and as we'd been joking about who would buy for whom the new Ian Rankin book, I'll try and look delightedly surprised on 25th.
One thing we've never done is buy each other expensive presents for Christmas or birthdays, a Terry Chocolate orange for my 50th and I loathe them, we tend to stick a £10 limit on it or buy a joint present for ourselves that we need for the house.