Don't rely on a jack when working under the vehical. Always use a jack stand and place it under the recommend jacking points specified by the car manufacture.
Don't attempt to loosen tight fasteners such as wheel lugs while the vehical is on the jack because it could fall.
Don't start the engine without making sure that the vehical is either in neutral for manual transmission or park for automatics and that the parking brake is set.
Don't remove the radiator cap from a hot cooling system, let it cool or if necessary using a cloth to cover up the cap and release pressure gradually.
Don't attempt to drain the engine oil until you are sure it has cooled to the point that it will not burn you.
Don't touch any part of the engine or exhaust system until it has cooled enough to avoid burns.
Don't siphon toxic liquids such as gas, antifreeze, and brake fluid by mouth or allow them to remain on your skin.
Don't inhale brake dust due to it possibly containing asbestos for older models
Don't allow spilled oil or grease to remain on the floor due to being able to slip on it and getting injured.
Don't use loose fitting wrenches or other tools which may slip and cause injury. Also you'll be looking for a new bolt or nut after that ones rounded off.
Don't push on wrenches when loosening or tightening nuts or bolts, its better to pull but if you have to push use an open hand to avoid scraping your knuckels if the wrench slips.
Don't attempt to lift heavy parts alone, always have someone to help you.
Don't rush or take unsafe short cuts to finish a job.
Don't allow children or animals in or around the vehical while you are working on it.
Do wear eye protection when using power tools and when working under the vehical.
Do keep loose clothing and long hair well out of the way or moving parts
Do make sure that any hoist used has a safe working load for the task at hand.
Do get someone to check on you periodically when working alone on a vehical.
Do carry out work in a logical sequence and make sure that everything is assembled correctly.
Do keep chemicals and fluids tightly capped and out of reach of children and pets.
Do remember that your vehicle's safty affects that of yourself and others.