Warm up properly.
Muscles work best when they are warmed up resulting in warmer muscle fibres created through increased blood flow. It is sometimes good form to do a warm up set of each exercise with a light weight. This also helps the muscles use nutrients, increases the oxygen supply and helps you lift more weight. I definitely recommend this fitness tip.
Time your rest periods:
Using a stop watch allows you micromanage your workout intensity. When working out you are trying to exhaust your muscle fibres and you don't want to rest too long. Short rest periods help keep heart rates up thus aids in burning more calories. Personally I rest between 90-120 seconds if lifting heavy weights, and 30-60 second rest between circuit training exercises. I will always train using a stop watch.
Lift to failure.
Whether you are training for muscular endurance, hyper trophy or strength make sure the last rep performed completely exhausts the muscle. Don't do more, don't do less. An exhausted muscle is one which is now ready to grow.
Take on protein.
Research has shown that instead of having your protein intake after working out, that sipping on a protein shake during training can speed up the muscle recovery process.
Stretch after training.
Everyone know hey should do this but often don't. Flexibility as discussed in this website is a key component of fitness. If you don't stretch, muscle fibres can get shorter whilst they repair.
I recommend you perform a stretching session after a gentle warm up on your day off from training.
Drink lots of fluids.
Muscles are up to 75% water and need to stay hydrated. Also if you are dehydrated your body will stop burning fat effectively giving the opposite effect of what you want post workout. Sip from a water bottle throughout the day. Don't gulp it down in at once.
Muscles only repair when you sleep. Although the recovery starts after you eat protein and carbs the actual muscle growth hormone will only be produced during sleep.
Alcohol, stress, poor diet will reduce testosterone levels. Testosterone levels are highest late afternoon time...so an ideal time to exercise.
To keep up improvements in muscle size and strength increase the amount you lift by up to 5% every 6 weeks. Record all the weights, sets, repss you are doing to properly micro manage your routine. Like anything in life. Do it right and you will get results. Do it wrong you will wish you dot it right.
Good luck in all your training efforts.