So, this is a REALLY late reply to this question from last year, but I figured I would answer in case someone else has the same question. 

For the most part, I just throw out expired coupons when I'm adding in new coupons each week.  I'm usually looking for a spot to put the new coupon so I just scan through the expirations and pull out any on that page/section that are expired so I have room to add the new ones.  Occasionally there are some that are missed so if I find a great deal while I'm at the store I always make sure to look at the expiration of a coupon I want to use to make sure I haven't pulled one that is expired.