Ozone generators and Nature 2 Spa Sticks are the best methods. A lot of people still use the old Bromine chemical system but who wants to relax in a "chemical soup". Some of the better spas available come automatically with Ozone generators giving virtually chemical free spa water.
These need to be built into the spa when ordering.
It is actually dangerous to bath in heated water above 38 degrees Celsius, because that is higher than our blood temperature and it can cause health problems. The recommended ideal is 35 degrees.
If you use the spa frequently and it's under 1500 lts use an Electric heater of around 5 to 6 kw. This will work every day at maintaining the water temperature. For over 1500 lts use an Electric heat pump.
If you only use it rarely, once every week or two, then use a gas heater or an electric heat pump, because they will heat very fast and then can sit idle till next time you want to use the spa.
These are not suitable for spas because they are a slow heat build up and can't heat a spa to any more than low 30's. They can not maintain the heat efficiently enough in a spa situation.
Blankets & Covers
Spas should always be covered. There are cheap bubble covers or foam blankets to hold the heat in or hard foam covers that form a child restraint.
Water Testing Kits
There are liquid test kits available but these can be a little complicated unless you are prepared to learn how to use them. A simpler method is the dip strip, 30 seconds job done. For the more adventurous there are CD Rom test kits available that have data recording facilities and treatment suggestions, just like having your own pool shop.