I got real clever early on. I got her a big notebook; and instead of cards (that'll get lost), every special day the kids write something and/or draw something in her book. She loves it.
Christmas, birthdays, Mothers Day, they're all in there.
Gifts are a pain, for Mother's day. The kids have a stall at school, so they get her something there.
This is a great idea and I wish I had started a book and earlier - it would be awesome to see the progression from baby to adult, adding their mark at all of these points.