I don't do a lot of grocery shopping on a bike, mostly walking and bus, but you are right that weight distribution is a key. I toured with 50 lbs split between four panniers on a folding bike and it felt very stable.

Trailer seems like a great idea for larger loads. You could also try those kind of shopping panniers and see how they work for you. I have this one: http://www.mec.ca/Products/product_d...34374302693353 , but I've seem other makes as well. This particular bag folds flat and secures with little velcro straps, so it stays folded. It also has a hard bottom part, which is great when you want to carry more fragile things and not squish them. Works better than a regular pannier for me.

You may want to start slow, shop more often and get less stuff. You will develop your own packing technique the more you do it.