Some of us do it because of the benefits - not because of the pay. You can make more than 14 an hour being a janitor (atleast in my area of the US anyway). However, the medical benefits here are worth their weight in gold. Once I become eligible (May of this year), I can shut down my medical coverage at my 8-4 job (and my wife can shut hers down as well)....that'll save my wife and I about a grand a month. That, to me, makes it worth it.
Loading trailers (I'm a twilight loader) is as hard as you make it really. My advice - drink water during the shift, even during the winter. If you aren't used to manual labor, tylenol, advil, or (my personal favorite) Alieve right before the shift helps a lot with soreness, as you will be sore during and after the shift. But, with time, you get used to it and your body becomes acclimated to the point where you may not need the tylenol or whatever your vice is before the shift.
That said, work at a pace that is comfortable. Going in all gung ho and busting ass is admirable - until you burn out about 2 hours into the shift and then need someone to jump in your trailer with you just to help you keep up. Better option is to work within your means. As you get "used" the demands of the job, the speed will come.
Other than that - as others have said - work as directed. Build walls as they want you to, use the load stand (if you need to), and try to keep one side of the trailer clear of boxes that fall off the roller line at all times (egress concerns). That's pretty much all there is to it.
Last, but not least - pay attention to the weather, and dress appropriately. If it's going to be really cold, a few layers are a good idea (remember you can always take layers off as you go through the night, start sweating, etc). Nothing worse than showing up in clothing not suited for the weather, and having to deal with freezing your ass off all night on top of having to do the job. I've seen guys show up in tshirts and shorts and say "well as long as I keep moving, I'll be fine"...only to end up hating life by the end of the night due to the volume / flow being light, resulting in them standing in an ice cold trailer with nothing to do except "trying" to keep moving in order to keep warm. Watching guys do jumping jacks just to keep moving is always hilarious.
Best of luck!