1) Tank Setup
TANK SIZE
Given, we should all know by now that in most cases, males should be housed individually, females should only be housed together if they do not show aggression towards other females, and males and females should not be housed together neither. With that said, I think the "one inch of fish per gallon" rule of thumb works pretty well with bettas. For everyday keeping of a happy and healthy pet betta, a minimum 1 gallon body of water will provide stability and buffering margins for water quality and temperature. You must be very diligent about your water changes if you decide to keep bettas in smaller setups such as bowls, holding cups and beanie cases. And you would also have to be very careful about monitoring water quality and temperature stability. The more stable your system is, the happier your betta will be.
I've heard stories, and personally witnessed, that a single male betta would "freak out" in a tank too large, such as a 10 gallon tank, if it does not have enough hiding space. Large open bodies of water would make a betta feel vulnerable, and he/she will get paranoid about being in the open with risks of being ambushed by other fish (if in a community tank). For this reason, I would say that a 5 gallon tank would be the biggest you should go for keeping one betta. If you wish to place a betta in a larger setup, make sure you give him/her enough hiding spots by providing a dense covering of plants (fake or live), caves and other similar ornaments.
SUBSTRATE AND DECORATION
A good layer of substrate provides colonization grounds for beneficial bacteria, that will provide biological filtration. Regular gravel is good enough for a betta tank. Since aquarium gravel now comes in almost all colors imaginable, the choice of color is all a matter of taste. No, I haven't heard from anyone about bettas freaking out from weird fluorescent colored gravel :p
As for decorations, there are all sorts out there from bubbling castles, to skeletons and mermaids, to plants of all sorts. Bettas will do well with any kind of decoration, and actually the more the better. One thing to keep in mind, is that to avoid sharp/pointy decorations that may potentially catch and rip the betta's long flowing fins. Castle roofs and needle-shaped plastic plant leaves are two examples to avoid. Many betta keepers will recommend using the soft silk-plants that are made of fabric. Another thing, bettas do need hiding spaces. So things such as caves and floating plant covers will make them VERY happy.
Personally, I adore live planted environments. I think they're naturally beautiful and they provide wonderful habitats for bettas. If you decide to go with a planted tank, you may have to be more careful about substrate selection, and pick one that's designed for planting, such as EcoComplete or Fluorite. As for plants, the choice would depend on your aquarium-keeping skill level and your equipment capability. For a minimum requirement planted tank that pretty much everyone can keep, stick with hardy low light plants. Anubias species (Anubias nana or barteri), java fern and java moss are few examples that you often find in betta tanks.
Temperature and Water Quality
Bettas are a tropical fish orignated from South Asia. Although they are hardy fish and can tolerate quite a range of conditions, they do prefer warm and clean, soft water. For everyday keeping, bettas do well when the water tempearature is between 75-78°F, and for breeding, 80-82°F is desired. Bettas do best in water of pH 6.8-7.4, close to neutrality. They also like the water to be on the soft side, at 5-10°dH, especially if you want to breed them.
When talking about water quality, there are two main elements to think about: chlorine and nitrogen. Chlorine can come in the form of free chlorine or chloramine, which are found in your tap water. They are added to the water supply by the city water treatment plant to kill any harmful bacteria. At the levels they come in, they are safe for human but can cause harm to your fish. Therefore, if you use tap water for your aquarium, make sure you condition the water beforehand, either by "aging" the water by letting it sit in a container for at least 24 hours before putting it into the aquarium, or by treating the water using neutralizing chemicals such as AquaPlus or Prime.
Nitrogen comes in four forms: ammonia, nitrite, nitrate and nitrogen gas. When fish poop and/or uneaten fish food degrades, ammonia is generated. This is EXTREMELY toxic to any fish, bettas included, and short contact can cause ammonia burn at the gills and longer exposures are fatal to the betta. For an established "cycled" tank, the beneficial bacteria (or nitrifying bacteria) will convert the ammonia into nitrite, which is still VERY toxic to fish. The nitrite is further converted by another beneficial bacteria into nitrate, which is tolerable up to certain amounts. Water changes are the key the removing these unwanted chemicals from your aquarium. If you have a established and filtered tank, the main thing you need to remove is nitrate, and you can do a 10-25% water change once a week. In case of emergency ammonia/nitrite spikes, use Prime or AmmoLock water conditioners to lock in the toxic chemicals, and do water changes promptly until you reach 100-200% water replacement. REMEMBER to precondition your replacement water to remove chlorine!
Lighting
Bettas aren't that picky about lighting. A regular flourescent or incandescent light, set for 8-10 hour light period would make them very happy. Just watch out the some lights tend to release a lot of heat. Check and adjust your tank heater accordingly to keep the betta tank's water temperature at optimum.
more to come...
2) Food
Dry food, freeze-dried, frozen, live
3) Common Diseases
Bacterial, fungal, parasitic