Stand as straight as possible, and keep your head up as you rake or mow.
Try to mow during the early morning and early evening hours, when the sun is not so hot.
Drink plenty of liquids to keep your muscles hydrated.
Protect yourself by wearing a hat, shoes, earplugs, and protective glasses.
Use as much as your body weight as possible to move your mower (unless it is self-propelled). This will minimize excessive strain to your arms and back.
When picking up piles of leaves or grass from the grass catcher, bend at your knees, not at your waist.
When raking leaves, use a "scissors" stance. This entails keeping your right foot forward and left foot back for a few minutes. Occasionally switch by putting your left foot forward and right foot back. Always bend at your knees, not the waist, as you pick up leaves. Make piles small to minimize the possibility of straining your back.