Just like everything else related to drumming; its all about conditioning. other than that, my recommendation would be:
1- get the harness to fit you correctly
2- wear appropriate shoes
3- use good posture
4- get use to it

I would like to know that same answer. I went to the Boston camp this year and tryed out for tenors. After the visual rehearsal I thought my back was going to break. Those drums were so heavy.

Donny

I agree, my high school had oldddddd harnesses for the snares, tenors, an basses. my back hurting so bad, it felt like i broke a bone or something. my back was like that for a week but i just had to get over it because it was band camp.

I don't think the pain completely goes away but having good posture can really help.

The best advice anyone has ever given me regarding back pain was a friend of mine during sectionals one Saturday...

"Quit bein' a little ho and suck it up."

On a serious note, I think what jowe said about posture and harness adjustment are what contribute the most to back pain. Make sure you're harness fits you, you stand up straight, and you're in shape. Also, focus on your playing/marching!! Not only will it make rehearsal go by a lot quicker, but the pain won't seem as severe.

I think the only pain you should feel on your back should be similar to your hands after a chop exercise. They should feel tired, but no sharp pains, and they should be sore for a few minutes after you stop. Any pain other than that, such as a sharp stinging pain or pain like someone has punched you should not be there if you have correct posture and good, working equipment. The muscle soreness should go away after a few weeks of good posture and workouts. Always stretch before and after tracking, or carrying your drums for a long time. If this doesn't eliminate back pain, then refit your harness to yourself and make it as snug as possible, to make the drums seem like an extension of your body. If none of this works, consult a doctor. BTW I got this info from my friend who marched corps, so it has some validity.

If you have back pain the use "Doans" back pills. They are over the counter and they work Great!

When you have back pain do these two steps.
1.Suck it up
The more you think about it, the worse it's going to get. Let your mind drift away from the pain and think about something else.
2.Stand upright
If you bend your back, it will only come back later. Don't ever bend backwards though, if you must bend, do it going forwards. This may look stupid, but it will save you from having back problems in the future. I learned this from the director at Texas A&M University. He experienced this personally, so he highly recommended this himself.

I would say take advil before and after a rehersal. It will make you sort of numb to the pain. Overall, you just need to tough it out and pretend its not there. Think about your drumming instead of your back pain. Thats what i usally do. You can block out most of the pain if you try really hard.