My dd is 5 and so far i've put them all in a box, but that's laziness and not being able to bear to throw them away. I actually have this year's still piled in our bedroom cos they came down when ds was born (to put baby boy cards up) 4 weeks ago and haven't moved anywhere sensible or tidy yet . I think I'll stick with the box idea - but i may need a bigger box .
I have a clip frame for each of the boys that always has a collage of the cards from their last birthday. They look fab in their bedroom, we look at them for a year and then they all go in a scrap draw to be cut up and trashed as they like.