Buy one nice gift so they are happy, and then spend the other half or more on stuff they NEED or would enjoy (slightly?) I always got new clothes, even things like socks and T-shirts are good filler items, its stuff they need. Then my parents always had one WOW gift.
Something else thats kind of fun that you could start is "look a likes" get them all the same gift, something that anyone could like, etc. Some of my look a likes have ranged from ridiculously loud toy gatling guns when we where 5 and 9, to HDTV's, to a bag of cheesesticks (Yes i had a christmas present in the freezer), and last year we got a box of cat litter.
Some years they are really epic presents other years they are just great laughs, either way I always remember christmas's because of our Look a likes.