Many rainwater harvesting systems are fairly simple and easy to maintain. Keeping your roof, gutters, and downspouts clear of leafs, branches, and debris is a must. If you employ any type of filter in your system, that filter should be cleaned regularly. Cisterns and storage tanks should be checked annually for leaks and in cold climates you may need to drain your tank for the winter to prevent freezing and expansion cracks. Storage tanks may also require cleaning at infrequent intervals.
More specific maintenance will likely be a function of your location and your individual rainwater system. If your system includes a pump for example, there may be periodic maintenance required. If your system is professionally installed, be sure to ask for detailed maintenance instructions.
If your rainwater system hooks directly into a sprinkler or drip irrigation system that may also require regular maintenance.