Your solar system is your friend. Spend a little with it. All solar systems require a little preventative maintenance. Keep a log of all maintenance on the system so future repair persons can see what's been done and spot long-term problems. The collector should be cleaned whenever it is dusty or dirty.
Water alone is usually sufficient.
Plastic glazing should be cleaned with mild soap and water. If the cover glass is broken, the interior absorber plate and insulation should be protected with a waterproof, opaque tarp that will not melt until the glass can be replaced. Glycol freeze protectants break down into acids with time and excessive heat. Check the pH, corrosion inhibitor and solution concentration at least once a year. Also, check the fluid immediately after it has been subjected to very high temperatures.