Due to space constraints, I am unable to have a drumset at home. Going to a studio would be optimal but I take at least an hour to travel to the nearest drum studio. How else can I improve on my drumming?
Bob Broadley is right if you want to practice ONLY the stick technique : buy a pad. So you will be able to train your touch and etc... which is nice.
Now, there are several things you can do if you want to improve your drumming in a more "evolved" way. First of all, buy a bass drum's pedal. This is always a good investment, since you will get used to your pedal and will train your ability with your foot. Then, what I did was to kind of create my own drum-kit, in a really simple way. For example, pans and stoves can be some relatively correct cymbals for training from time to time, the practice pad can replace the snare and etc... Find some things that remind you the touch of the different component of a drum_kit, then organize them in the way you like.
This won't replace a real drum-kit AT ALL, but at least you will be able to work your technique, try to find some good rhythms and figure out if it can be playable for you or not.
The most cost effective method is pillow practice (or alternatively hit any object that makes the stick bounce easily although the lesser it bounces the better your technique will be with making up for lost motion.) practice paradiddles and any song riffs you know and even listen to new songs / genres to get a wide variety of techniques and progressions.
If money is not a concern practice pads and even small electronic drum kits are a great help. practice where every you can. Tap feet on the bus or traveling long distances pat your hands in a rhythm on your knees, although drums are the biggest instrument to provide space for you can still carry it around in you mind :).
Also if you are adept in crafting you may be able to pull off building your own Practice pad kit. I am planning to get three practice pads and mount one as a double kick pad and add a snare and hi hat pad in their right places. Worth a try!
There's a guide, in Portuguese called Baterna http://aurelio.net/baterna/ (sorry, I am from Brazil, and this thing is the perfect answer for your question) that teaches people how to play drums without drums, if you can translate it with Google Translate or something, it is worth a read.