1. Understanding yeast better. For me, this ranged from understanding the subtleties that are imparted by particular yeast strains to "temperature control". I say "temperature control", b/c I still only have active temperature control on my lagering fridge. For all of my ales, I use other means if I need to lower the temperature a bit, such as a jerry-rigged swamp cooler (wet towel with carboy immersed in a bit of water, fan nearby if needed). This is all mainly to say that temperature control doesn't necessarily have to be automated if you're on a budget and/or have limited space for additional fridges, etc.
2. Creating my own recipes. Exploring recipes on your own - using input from various other sources, but trying different grains, hops, yeast, etc. to get a feel for the subtle changes imparted by each ingredient is a great way to improving your brewing. It takes some time, so you have to have patience and be ready for some beers that don't taste quite how you wanted them to, but you will improve.
3. Making simple beers. Even when creating your own recipes, there can be an impulse to make BIG BOLD beers, and often this is great, but this can also lead to some BIG MUDDLED flavors and hitting a rut in terms of your understanding of how small changes can impart relatively significant differences in flavor, etc. If you make a 10% imperial stout with vanilla and bourbon soaked oak chips, follow it up with a pilsner, an alt, or a simple pale ale to get back to basics and make sure you can still nail some simple (but difficult to perfect) recipes.