Buy a better quality rear rack, like the following:
http://www.peterwhitecycles.com/ortlieb.asp
http://www.peterwhitecycles.com/tubus_racks.asphttp://www.peterwhitecycles.com/index.htmlThis can carry more than a gallon of milk in EACH panniers (The Panniers are sold as a PAIR, one for each side).
If you have about $900 dollars I would recommend a trailer:
http://www.cycletote.com/I had a cycle-tote trailer which I have used for many years. It was easy to haul and easy to attach when needed. I have A BOB trailer, which is a lot cheaper, it does NOT haul as well nor attached to the bike as well as the Cycle-tote. The problem is a Cycle-tote cost $900, but a BOB is about $300.
http://www.rei.com/product/722504Now the BOB is the better trailer if you go off road i.e. on a path through the woods NOT a bike path or even a dirt road, I mean a path through the woods. For Bike Paths and even dirt roads I would recommend the Cycle-tote, but on true paths the BOB trailer comes into its own.
If you really want to see what a bike can haul as a trailer:
http://www.bikesatwork.com/hauling-cargo-by-bike/hpv-cargo-capacity.htmlNow if you have a Lap top, you may want the following:
http://www.ortliebusa.com/cartgenie/prodInfo.asp?pid=140&cid=2I do own one of these, it holds my laptop quite well, if and when I decide to take my lap top with me.