Hey mate you will find this a very personal thing I am sure. But here is what I recommend. 2 warm up sets of around 50-60% of the weight that you will use as your max. But the warm up is only part of the problem mate. You can be using strict form but its also the tempo of the rep that can cause injury. The faster the rep the more stress transfers to the conective tissue and can cause injury. Think of your tendons as rubber bands that take loads. The smoother the transition is between the concentric and eccentric (up and down) part of the rep, the lesser amount of stress will be placed on the tendon. So smooth controlled rep speed is possibly more important that warming up.

never noticed a diff if i warmed up or not.. Just if I used correct form but doing rotator cuff exercised before a chest or shoulder work out can do wonders.. Also keeping traps in on pullups etc

I always do 2 warm up sets of 15 reps of medium weight for mostly every exercise i do, then bang out 3-4 heavy weights mainly always going to failure..i rarely stretch before i workout mostly after or maybe between sets,,if anything i flex the muscle im workin on to get more blood in it.lol

Few light high rep sets. Pyramid lifts help too. Pyramid style example
15 @ 150
12 @ 175
10 @ 200
8 @ 210
10 @ 200
Etc. Higher in weight less reps.

i like to stretch first. then use VERY light weight (high reps) after i start to break a sweat i stretch again. if im not feeling any tightness, then im very to go. on shoulder day it takes me longer because my shoulder are very sensitive for some reason

i start with a few high rep sets making sure not to fatique the muscle then a couple relatively heavy warmup sets to get your body ready to overload the targeted musle. once youve warmed up a certain muscle there is no need to warmup for its second exercise.

I had a shoulder injury a few years ago and now I can't stress how important a good warm up and proper form is. I feel like I preach to people to say fuck their ego and lower their weight to get proper form and take momentum out of their lift.
Momentum = injury

I usually start jumping rope then warming up rotator cuffs for every single workout. Then for chest I will do about 30 bodyweight reps, starting low and workng my way up using all different grips. Then a warm up set with what ever exercise I start off with.
another thing you can do when you work anything upper body or legs is to get on an eliptical for a couple minute and power throught it with your arms and upper boddy, it's a good way to get the blood flowing and to warm up your joints.