I use a binder to keep track of everything, so that I don't have to keep it in my head.
One of the sections in my binder is labeled GIFTS. I write down what gifts we gave to whom (just the major ones-- kids, grandparents, cousins) for Christmas, birthdays, Mother's/Father's Day.
This helps to:
*make sure we don't give the same thing twice;
*give along the same level from holiday-to-holiday (ex: It is bad to give the mother-in-law a mediocre "our-life-was-too-busy-to-really-think-on-this" gift for Mother's Day, and then give the father-in-law an amazing whopper of a gift a month later;)
*remember ages we gave a "biggie" gift to our kids, to give it at the same age for the other kids;
*remember the "traditional" gifts that we give every year (I would forget if it wasn't written down!)
I also have a page in this section for gift ideas. If someone mentions something they like/need/want, I am reminded to keep a look out for that item.
What tips do you have for keeping track of gifts?