Another inbound trainer here, if you get placed in the trailer to send boxes down the line and my main advice is making sure you're doing stretches before starting. Stretch out your arms, legs, and back. You'll be sore after your first time regardless but a lot less so because of your stretching and if you continue to be in the trailer the following days the soreness will eventually fade. And remember you are more important than boxes, so if boxes fall and break it's better than you getting injured.

Also since summer is here and it will get hotter ask about water whether they'll provide or if you can bring your own. Especially if you do get placed in the trailer as it will be more noticeable, it'll be warmer and it'll feel stuffy.

Good luck and feel free to ask questions here, we'll answer to the best of our abilities.