Another topic got me thinking of the importance of this. Always take some time to stretch before loading/unloading your kayak. And don't forget about your legs. I have sciatic nerve problems, knee problems, and a crappy back. I always find myself hurting a few hours into a trip, so here are a few tips I think will help almost anyone prevent injuries and have a more enjoyable day on the water.
1. Drink plenty of water before, during, and after a trip. Not just beer, water too guys...
2. Take 5 minutes to "warm up" before you do any lifting. Stretch your back, legs, arms, and everything in between. Maybe do some air squats or lunges.
3. If it is available, use it. I always offer to help people load or unload when I can, as do most of us. It helps more than some might care to admit.
4. Make sure you have a solid grip before committing to a lift. No one likes to drop anything, much less a kayak.
5. Take your time! No need to rush and get hurt.
6. Paddle to a shoreline and take a break every now and then. Stand up and walk around for a couple minutes. I retie and eat snacks when do this.
7. Always bring some sort of food, even if it's just some snacks.
8. Drink more water.
9. Make it a point to lift with your legs, keeping your back straight and core tight.
10. If you have an injury, be extra cautious about the smallest of movements. Tweaking a bad back is easier than some think.