As everyone has recommended, a fermentation chamber is a good way to improve your brews, but can be challenging to build. I tried to build a Son of Fermentation Chiller and failed miserably (my insulation was warped and I didn't have the proper tools to measure or cut the insulation very well). Here are my recommendations which you can implement immediately or the very near future:
1) Place your carboy inside your mash tun (remove the braid and barbs -- replace the original spigot). Fill it with cold water. Cycle out the water whenever the temperature gets too warm. With a 6 gallon Better Bottle there's only space for about 1.5 gallons of water in the cooler. When I refresh the water I'll usually cool about half that volume in the fridge, then drain the cooler, mixing half of the volume in with the new chilled water (that way you don't drop the temperature too much very suddenly). I also usually cover the carboy with two towels to help insulate. This way I can maintain about 65 degrees if I'm vigilant, or 70 degrees if I'm more relaxed about it. For reference, this has been during the summer when my apartment has averaged 80 degrees. The temperature obviously swings a bit because of refreshing the water, but I've observed no ill effects.
2) Buy a pond pump. Someone else recommended this too. You can use it to circulate water through your wort chiller, and even better you can circulate chilled water (around freezing is ideal) when you are approaching pitch temp. My tap water is currently just below 70 degrees, so if I want to chill to 65 then I need something colder. My pump can push 400 gallons per hour at 0 head pressure, and I wish it had a bit more power, but it is adequate (and cheap, about $20).
3) Build a stir plate. You can do it in about 15 minutes (plus down time waiting for glue to dry) for about $5-$15 (or even for free) depending on what sort of features you want and whether or not you have access to free magnets and computer case fans. There are many good guides to doing it online. I have a blog entry about it, although it isn't necessarily the best guide. If you don't have time to build a stir plate, give your starters a periodic swirl or shake now and then. I can't find the blog that posted it, but I remember seeing a table where swirling/shaking the starter produced decent results, even compared to a continuous stir plate. I've also read that vigorous aeration after the pitch is adequate and that continued stirring is unnecessary, but obviously that depends on how much yeast you're trying to grow. Just be smart about starters.