I recently had a problem with my messenger bag. I purchased the M51 engineer's field bag a little while back to replace my former bag which suffered catastrophic strap failure. The field bag was actually an excellent bag, for awhile. Anyone who has read a review of this bag knows that Rothco, the company that reproduces the bag, is notorious for bad stitching and each bag is hit or miss as to whether or not it fails. Part of it is wear based, how hard you are on the bag. For me I carry about 10-12 pounds to school and back each day, which is about a 2 mile trip. Not that far but the bulges and size of stuff I was carrying caused the latch to pop off, and the cloth flaps which held the anchoring rings for the strap began to fray until one almost failed. The latch is a separate problem that I still have to deal with, but the strap needed to be fixed fast. In the reviews I read everyone said they broke out the sewing machine and restitched the flaps back together. I know nothing about sewing so I used the mechanical knowledge I do have.
My solution is a bar inserted into the bag and bolted on to the original thin bar that held the top rigid. The new bar supports the strap and takes the full weight of the bag. While it was designed with the express purpose of fixing my bag most messenger bags are similar enough that the solution can be adapted to another bag.
Lest I have discouraged you from the m51 bag let me say this: it is an excellent bag which has served me exceptionally. So far only the latch and strap have failed, everything else has been fine and has replaced my former bag fitting everything in despite being half the size, and I still haven't filled the map pouch or the all of the back pocket. It is comfortable, well sized, and filled with more pockets than I have ever seen. Add this to being cheep and it makes a bag that is absolutely worth having.