Bending and kneeling: what not to do: when you are weeding or you are going to lift something, don't just bend at the waist!
Bend your knees. It's worth repeating: bend your knees!
When lift something off the ground, get in the habit of squatting down, balancing your weight through your heels, engage your abdominal muscles, and use your thighs to push yourself up to a standing position.
If you weed from a standing position - bent over - still bend your knees. If you are yanking out stubborn weeds you could easily strain your back. Absorb some of the strain with your knees. Try resting one arm on your thigh with your knees bent as you weed from this position and then switch arms. Weed ambidextrously! After ten minutes stand up and try these stretches for your hands and arms:
Extend your right hand out in front of you, pull your fingers back toward you, and stretch through the palm. Do this on both sides. Then interlace your fingers in front of your body, reach up high, lean gently to the right then the left. Then with your fingers still interlaced, push out in front of you and round out your upper back.
If you can, clasp your hands behind your back and squeeze your shoulder blades together and down. Open up your chest. Take three deep breaths.
Drop your arms down to your sides. Make a fist with each hand and turn them so they face to the back and the side. Curl your fists upward and then backward in each position, rolling your fists and stretching your forearms.
If you kneel when you are weeding or digging with a hand spade in the garden, don't kneel on both knees. Keep one of your feet flat on the ground - this will give your back more stability.
There are lots of little gardening stools and seats available, but I've never been able to use one. I move around too much, a stool just gets in my way.
I do like a kneely-pad, especially when the ground is still damp.
While most gardeners tend to wear their old, ratty shoes while gardening, stability and support from the ground up is very important for moving properly and staying injury-free. Your feet should feel supported and your ankles shouldn't wobble. Make sure you have a comfortable pair of shoes with good arch support in them just for gardening.
(Do not garden in flip flops, please - I beg you.)