I haven't bothered to keep track of sources, but everything seems to point towards the same conclusions.

• Strength gains go way up with the level of stress on your muscles (but unfortunately, so does the chance of injury). Lower reps, extra reps with spotters, negatives, and especially constant velocity machines are good for this.

Getting sore is a decent indication of how stressed your muscles were.

Studies have shown that more than 1 set doesn't help, but that's only because they use people that haven't been weight lifting and their muscles get plenty torn up after 1 set. More sets can help if you've been doing it for a while.

More reps doesn't build strength so much, but will add more energy storage to your muscles so that you can do more reps (bodybuilders optimize towards building muscle in this way)

• at first, it takes very little time to get way too sore. I only spend about a half hour at the gym, but since I lift heavy weights/low reps and don't waste time (keep my heart rate high), I can get a full workout and be sore all over. I also find it more fun this way.

You can hit diminishing returns after lifting one muscle too much in one sitting or too soon after lifting, so just use soreness as a rule of thumb and don't exceed either of those bounds.

• Creatine, and depending on your diet, protein. We're already almost full of creatine (unless you're a vegetarian, then even your brain suffers), but topping off can help push out a couple more reps and stress your muscles a bit further.