You're not doing a thing wrong!
Ammonia is the final product of metabolism from all life in the aquarium. From your original post, you don't have much of a bioload (net metabolism of all tank life.) It could be that you're coming to the end of your cycle, which isn't necessarily a bad thing. There are a few types of bacteria. One type changes ammonia to nitrite, another nitrite to nitrate. From the beginning, the first type of bacteria will colonize to neutralize the ammonia. Then the second type will colonize to nitrify the nitrites into nitrates.
If you don't have a huge source of ammonia to start with; the ammonia can peak, and then the next type of bacteria begins to colonize to do its job. Your stated ammonia value of .5 might have been the "spike." Remember, success isn't dependant on a huge spike in ammonia levels.
The important thing is that all necessary bacteria colonize and nitrify their respective compound to the point of equilibrium. In other words, there will be an ample population of bacteria to rid whatever ammonia and nitrite is produced in your specific tank at a rate where ammonia and nitrite will remain undetectable. It remains undetectable because the ammonia and nitrite is taken care of as it's produced--that's equlibrium.
You'll know this has happened when your ammonia and nitrites are zero, and nitrates become detectable. Given time, this WILL happen! Your only concern (having cycled your tank with a scant ammonia source) would be to make certain you add livestock slowly. You don't need a huge population of bacteria present to rid your tank of ammonia. Each time you add new life, the existing colonies will grow to equal the new load.
Like Luna said, be patient. This process has yet to fail! A watched pot never boils, you know! Go out to eat or something!
