Tip #1: When lifting anything, whether it's 5 pounds or 50 pounds, always make sure to bend at the knees and hold the object as close to your body as possible. This will help prevent from "throwing out" your back. OR, if you have bad knees, get some youngster to do the lifting for you!
Tip #2: When gardening, switch positions often. When able to, sit in a chair or work at a gardening table/bench. When you need to get low, you can sit, kneel, lie down, etc. The key is just to not stay in one position for too long.
Tip #3: Make your best effort to keep a straight back. The best way to do that is to try and keep your shoulders pulled back as much as possible. This will not only keep your shoulders and upper back straight, it will also help keep your head over your shoulders and your low back straight and tall.
Tip #4: Use tools. Any tool that has ever been invented was made to make your job easier. It's the definition of a tool. Use a shovel, hoe, rake, trowel, etc. whenever possible. It will put less strain on your back, shoulders, and arms.
Tip #5: Ask for help. Invite people over to have a gardening party. This way, you'll get to hang out with your friends, but more importantly you'll have assistance when needed and get the job done faster.
Tip #6: Be sure to stretch thoroughly before and after yard work. Keeping limber, lean muscles will help prevent any tension and soreness the day after.
Tip #7: Drink like a fish. Staying hydrated is key in any and all situations, but it's definitely important when outside and exercising. Water helps keep your joints lubricated, your muscles pliable, and your mind headache free. Always have a water bottle within reach when you're in the yard.
Tip #8: Get adjusted. The best way to ensure lack of soreness and back pain from doing yard work is to visit your chiropractor beforehand. He/she will be able to get your spine in line before doing any back breaking labor. And, of course, if you need to see a chiro after your yard work, you're more than welcome to do so.