So I'm having a bit of trouble schlepping my stuff on my commute. Looking for tips.

Here's my "situation" --

I commute from my home (room rented in a house) to a university. At first, I tried bringing my laptop, papers and books in a Timbuk2 Commuter bag, which worked fine, but tended to get pretty full. Additionally, it had a tendency to slip down to my right side while riding. I also try to bring my lunch in a Zojirushi upright bento container (Mr. Bento) which comes in a bag with a long strap that I wear like a messenger bag. This really, really tended to slip to my right side, and down in front of me.

So I switched to my Ogio Metroid backpack which, so far, has worked a lot better. More capacity, no slipping etc. However, I'm also bringing my small gym bag with me. While wearing my backpack, I can put the gym back over my shoulder, but again, it tends to slip down and I find myself hitting it with my right knee with every pedal.

I own panniers, but that means I have to take those with me wherever I go during the day after locking my bike, and I tend to move around campus a lot during the day. No real "office" to speak of. I also have access to a locker in a central building where I put my gym back at the beginning of the day, and switch out with my backpack when I'm going to the gym.

Still, this gym bag-on-the-bike thing is a pain. Any tips on wearing it or getting that stuff to campus more efficiently and conveniently?

Bungee cords and a rack?

Does your TimBuk2 not have the side strap? That should help keep it on your back...

Otherwise, a net bungie maybe?

Perhaps you might consider a basket.

I think lambo_vt's suggestion about a rack and bungee cord for either your backpack or gym bag should work fine. Also, see if your school gym, art department, or some of your older buildings on campus have lockers in the basement or first floor. You may also want to look into a larger backpack. If you are a student ask a professor you like if you can keep some stuff in thier office. You may also want to find a professor who commutes as they would understand your plight. You can also reassess what you need to bring (i.e. computer when computer labs are availble, or thumbdrive for data that you need).

Bring gym clothes in batches so you don't have to carry that bag every day. Like a week's worth at a time.
Lose the bento box and bring your food in a plastic grocery sack. It packs easier (fits in any available cranny) and weighs less.

I've been using one of the Rivendell basket bags for a few weeks and it's made carrying items to and from work very easy. It's worth a look.

This is why I have a Topeak basket zip-tied to my rack. Holds my backpack and keeps my back free, holds a 30pack of beer too!

A pannier-bag is a good option to replace the messenger bag. Mine holds my laptop, power supply, blackberry, lunch, raingear, lights, pump, tube, toolkit, technical manuals, a thick paperback and a travel pillow for the train ride. I've got a cheap rolltop Detour Uptown pannier that's OK - impervious to rain, has a notebook sleeve and reinforced rubber bottom, goes on and off the bike quickly. It doesn't have a shoulder strap (I've improvised one), and the mounting hardware is odd and awkward when carrying it around, and the rolltop means there's no quick access to the stuff inside, tho it does have mostly weatherproof pockets outside.

Carradice, Ortleib and Arkel make very, very nice briefcase panniers that hold a ton of stuff... they have mounting hardware that's hidden when off the bike, and built-in shoulder straps. They are pricey, tho. I plan on getting the Carradice as soon as I set aside enough "mad money" to cover it.

A nice Wald basket, up front or out back, is only $30 or so, and you can toss in your bags and bungie them down. If you don't mind the weight or the Fredness of it all, the wire basket panniers/rack with a pack of dollar-store bungie cords are awesome for lugging stuff.

Hey, I resemble that remark! Embrace your inner Fred, chicks dig it