I love couponing in the US! I am sad that they're not readily available here the way they are there, because I used to get all my toothpastes, deodorants, shampoos etc for next to nothing.
I normally do like CaliGirl, and go for the clearance items. There's a tesco around the corner from where we live, so we go almost everyday, after the 2nd or 3rd round of reductions have been made. Normally we can get some sort of vegetable for around 10p, or a loaf of bread for 17p. Not an amazing deal, but yesterday, I got some reduced 500g Organic beef for £1.43 instead of £2.87; also got some hummus for 30p. In my area, I've found tesco to have better reductions than sainsbury's, marks & spencers is hit or miss, and I haven't tried waitrose (though I might have to!)
Also, the only place I've seen some grocery coupons was in the Sun, they were just part of the paper though, no special insert. They were specifically for Tesco--£1off when you spend something like £4 on fruit or something. I think it might be a weekly thing (like wednesdays or something), but I'm really not sure. I also saw a Catalina on the ground yesterday from Sainsbury's, but didn't bother to pick it up, so I'm not sure what it was for.