squats:
chest up at all times
drive up with your hips (don't push at the ground with your feet as though you're on a leg press)
maintain a straight back (no arc in the lumbar spine)
keep your hands as close togethor as you can so that the barbell sits on muscular mass rather than bone (notice that the back muscles/deltoids form a little "pocket" when close togethor)
dont use your thumb in the grip of the bar (i.e dont use the same grip as you bench press). this puts unnessecary pressure on your elbows and shoulders. rather, place your thumb over the bar instead of under it
ensure you squat into a position where your hips are paralell to the floor (or deeper). this relieves pressure off the knees
lifting belt will help with lumbar support. use a belt that has a regular width throughout
look at a point somewhere on the floor. do not look up (try squatting whilst looking at the ceiling, you can't)
deadlift:
ensure the barbell follows a vertical, straight line path
chest up
maintain a straight back
deload the barbell
generally:
buy (or download) rippetoes instructional videos (5 hours worth) and his two books. i downloaded them recently from a torrent on TPB
don't lift more than you can. think realistically, you arent going to go from curling 12.5kg dumbells to 15kg dumbells in 2 weeks (i.e. 20% strength gain)
don't be a hero
don't get your back into it regardless
this isnt really a safety tip, but avoid using the smith machine. its too restricting (e.g. denies the natural arc in your bench) and you fail to develop stabilisation muscles. xkg on smith machine =/= xkg free weights