try a cheap fix first. Tie wrap a milk crate onto your rack, toss in your backpack and ride for awhile. If it feels better invest in some paniers. I've been doing the milk crate since June and it's kinda like having a pickup truck. I just toss stuff into the back, if I start to overheat on a ride I can peel a layer at a red light and stuff it in the crate. Too much weight, and it makes the bike a little wobbly.