Unfortunately, I'm not an expert, just someone who's gotten good at using Google to quote other experts. More unfortunately, the majority of posts I've found are from physical trainers extolling the "benefits of manual labor" as opposed to a gym and a lot of testosterone-loaded accounts of "I worked for a summer in the pineapple fields and that made me the man I am today. Then I went back to my air-conditioned office to do some filing". Fortunately, there are a few accounts of people discussing what to do to help with manual labor.
Proper diet and sleep
First off, you will want to eat a lot and eat healthy. A manual laborer can easily burn around 3000 calories in their shift, and if you're not getting sufficient fuel, you're going to be in worse shape. There's probably going to be a lot of carbs to give you the energy you need, but don't skimp on protein (good both for the muscles and the joints) and fat (good for energy). Try to avoid adding too much sugar, although don't worry too much about eliminating it since you are working off those empty calories. As for sleep... well, that's a Catch-22. You need more sleep, but after a 12 hour shift, and probably an hour or two commuting, combining with the usual exigencies of life, you probably don't have the luxury of sleeping a lot, even once you get past the aches and the pains. Still, at least do your best to avoid shorting yourself.
I know... you've already done this exercise. Why do more? And where do you get the time? The common thread in this conversation thread among those who were stating that they'd been doing physical labor for decades was that they added additional weightlifting and cardiovascular exercises. As best I can tell, it's a matter of that your typical day of labor works only the muscles you need, and overworks them. The additional workout helps strengthen other muscles that will help support those joints, and help build flexibility.
Unfortunately, this is where my ignorance means I'm a bit at a loss. It sounds like most of these people were just doing the standard hour of regular lifts with the following caveats:
- Ease back on exercises that involve the muscles you're using in your labor. They already got worked out, and you don't want to impede your work performance. Do a few light sets with them -- the light exercise helps condition those muscles to recover -- but don't overload them.
- If you're also looking to build muscle, delay those exercises for just before the weekend to allow recovery, and you may need to boost the weight just to get high enough to shock your muscles into accepting that they're actually doing more work.
- Don't neglect stretching, but do it after your exercise. Part of the stress of manual labor is that you're doing all of that work at odd angles, which means flexibility is key. And the stretching at the end of the workout will, much like the light exercises, help relax the muscle and get it into repairing itself. One tricky thing here, you may not be able to rely on your sense of pain (you're probably conditioned to work through it), so go easy, and consider relying on the sense of tightness instead.
Consider investing in a foam roller
I'm not too familiar with these myself, but this guy suggests it in his essay on the perils of physical labor and why it's not glamorous exercise. Supposedly, they really do help in easing stiff muscles so that you can move again.
Like I said, I'm not the expert you need for the job. Hopefully Dave or one of the others more qualified to state exercises will chime in.