Velvet
General Info and Prevention: Velvet, another parasite, is very treatable when done promptly. It can be prevented exactly the same as how ich is. Fish have a greater chance of contracting velvet when their water temperature is below 72 for extended periods of time, and also if the water quality is bad (just another reason to invest in a heater! They're not that expensive). Aquarisol (see above) is a great preventative of parasites like Velvet and Ich.
Symptoms: Your first hints that your betta has velvet will probably be that he is simply acting out of sorts. If this is the case, find a flashlight and shine it at different angles over his body. If he has velvet, he'll have a gold or rust color all over his body. He may also have clamped fins and be scratching, or have lost his appetite and color.
Treatment: If your betta lives on his own, do a complete water change. If he lives in a community tank, do a 30-40% change. Ideally you would have medications on hand, but if you don't, dissolve some aquarium salt to add while you get medications. Many medications that treat ich are also good for velvet because they are both parasites. That said, try Aquarisol (see Ich, above, and the First Aid page), but some other good ones are Maracide, Malachite Green, and Coppersafe (as well as other copper-based medications). Just remember that copper is poisonous to bettas in large quantities so you must dose copper meds carefully. Like Ich, you should be performing water changes in between treatments. Do some research about the medication you plan to use first, because it could be harmful to live plants or invertebrates that you have in the bowl or tank. When you medicate, heat your aquariums slowly to 82-84 degrees F to speed up the life cycle of the parasite. It may also be helpful to keep the tank in a darker room: velvet is a parasitic algae and needs some light to survive, so so you might want to deprive it of light.
After treatment, keep an eye out for secondary infections. Your betta will be more vulnerable to other illnesses now that he has been weakened by Velvet.