1. It is generally a good practice to purchase the largest aquarium that you can actually afford. A larger aquarium will be less susceptible to quick changes in the chemistry, and will offer much more stability.
2. When adding new species to your aquarium, you have to ease them in. Adding too many fish too quickly can cause the ammonia and chemical levels to raise quickly. Adding fish slowly gives your filter time to catch up.
3. Having a quarantine tank on hand is a very good practice to use if you frequently buy new fish. The quarantine tank will weed out any sick fish that could possible get into your aquarium and cause harm to the other inhabitants.