I was going to say The Krazy Coupon lady too... she has a blog that I link up in my Google Reader... she has instructions for the hard core.
I don't have a place around me that doubles either, well, Copps/Pick 'n Save has promotions once in awhile... so not much help either. Also the cheapest grocery store in my area does not accept coupons printed off the internet.
One word of advice that I got from a checker is that if the internet coupon isn't going through when they ring it, to have the checker cover up the longer UPC code... then it will scan.