Advice for Rainwater Collection

If your tank is going to be large, consult a builder or engineer, as it may require structural support.
Have the tank installed by a plumber or the tank manufacturer. This will ensure the system operates efficiently, and is easy to maintain.
Lead-based paint and flashing, or tar-based coatings found on some roofs effect water quality. Make sure your roof material is non-toxic.
A u-shaped gutter traps leaves and twigs. If possible, choose water friendly gutters, or cover existing gutters in mesh.
Try to make all pipes 'dry'. This means having a continuous downward path so water can't accumulate in them between flushes.
Have all openings covered by a mosquito-proof mesh.
Install a first flush diverter on the downpipe. This is especially important in areas of high pesticide use or atmospheric pollution.
A sump box between the downpipe and the tank can slow the water flow down, separating out any sediment no previously diverted.
Make sure the tank overflow outlet is connected back into the stormwater pipe or irrigation system.
Make sure tank water is aerated, so it will not become stagnant.
Clean the inside of the tank every few years, as sediment will accumulate.
You may need to install a pump to provide adequate water pressure for some appliances. Consider appliances that can operate at less than mains pressure.