What are some basic "DON'T"s?
ANSWER Don't sit your terr where it will get direct hot sun. Temperatures in a terr can get very hot very fast killing anything you have in it.
Don't use tap water if you can avoid it -- it often is high in minerals and chlorine. The minerals build up as the water evaporates leaving the minerals behind. This eventually can harm the plants roots. City water often contains chlorine to kill germs. This isn't good for many plants. If you must use tap water that has been chlorinated, let the water sit out for a day or two to give the chlorine time to evaporate out. Whenever possible use rain water, distilled water, or RO [reverse osmosis] water.
Dont get water happy. It is much easer to add a little water to a tank that turns out to be a bit too dry, than it is to try and get an excess of water to evaporate before the plants contract root rot. With the exception of bog terrs, an occasional misting is all your setup will likely need.
Don't fertilze if you're using soil often unless you plan on replacing the soil frequently. In a terr, mineral salts can quickly build up and kill the plant roots. Many small bottle gardens and terrs will never and should never be fertilized. Just make sure you start with good fresh media that is appropriate for you plants.
Dont use the charcoal brickets made for grills in your setup. Such coal is treated with chemicals that will likely prove detrimental to your plants as well as any animals you have in the tank.
Don't crowd your plants unless you want to spend a LOT of time trimming them and replacing those that get too big.
Don't pick plants that get big quickly. Same reason as above.
Dont use perlite, any sort of crushed walnut shells, or cedar chips in your setup, if you decide to include animals in your setup. The first two items may get ingested by an animal causing a gut impaction. If this happens, the animal will die. Cedar contains compounds that are lethal to many invertebrates and can harm vertebrates as well just from its vapors and by direct contact.