Stretching is good for immediate and long-term injury prevention. Stretching can play a large part in increasing your distance by keeping you loose. Tight muscles are slow and conversely relaxed muscles are fast. The more flexible you are the more your can coil and build up energy to release and thus more clubhead speed. One can not swing too fast.
Compile a record of your statistics from your rounds and look for a pattern of weakness. That could be too many putts, penalty shots, poor strategy, etc. Assessing the weaknesses in your game is the first step towards improvement. Success comes from having a good plan that targets your weaknesses or mistakes. Remember golf is a lifelong sport and you have the choice of staying at your current level or working to get better. Practice to improve your weakness.
Have a plan for each practice session. Make your weakness your strength. It doesn't make sense to practice what you are best at. Seek out a qualified instructor to help develop a plan for you. Correct your flaws and spend some time practicing each aspect of the game.
Take a break after 30 shots or so. This way you can stay focused and not fall into a slump. Watch that you don't follow up a few bad shots with more bad shots by hitting more range balls. If you find yourself struggling, slow down, concentrate on making a better practice swing, then try to recreate that with the ball. Golfers have dominant mistakes or tendencies and when you get tired your muscles will go back to those dominant habits. Your progress can drop towards the end of your practice session if you start to get tired. Now we are taking a step in the wrong direction erasing any progress we may have made.
When you hit range balls you should be concerned with your technique and ball flight and not distance. You don't play the same ball as they have on the range. Range balls can be heavier than legal balls and hence may fly farther. Also, range balls are cheaply made and lack consistency. Exposure to the elements and temperature changes will surely affect the liveliness of the ball. Each ball is probably of different age and the wearing down of dimples will affect the consistency of the ball flight. The yardage markers usually are not accurate and probably not measured from where you are hitting from. Focus on ball flight and your technique.
If you are working with an instructor on some changes in your swing, stay committed to those changes. It can be easy to lose the feel of what you are working on. If this happens don't abandon what you are working on and start trying other things. Tips that do not target your mistake will only make your practice more difficult and will also lead to frustration.
Notice there are fewer don'ts, we want to be positive. Thinking about what we want to do rather than what we don't. The mind doesn't recognize negatives. When we think about where we do not want the ball to go, that is usually where it winds up. Start thinking about what you want to accomplish, rather than what you don't want to happen. If your plan is correct, the lower scores will come. Be patient and stick to your plan the results are worth the effort.