1. Practising tactics – I cannot stress enough the importance of tactics at the sub 1800 level. If your time is limited, tactics training would be the one thing that would give you the greatest return on investment.
2. Playing and reviewing my games – This allows you to learn from your mistakes, and to prevent you from repeating your mistakes over and over again. This important training tool provides you with the appropriate feedback loop, so that your chess training is focused on those areas that you need to improve the most.
3. Revisiting my thought process – Learning more about how other chess players think, and then taking an inventory of my own chess thought process. I documented a process that made sense to me, and then reviewing it while going over positions and / or annotated games.
Feel free to add a comment of your own list of things that have improved your game.