In-line filters, quick couplings, pivot couplers and foot valves. The plumbing between the tap, the pump and the gun. An in-line filter on the inlet is the cheapest pump insurance there is - grit is what destroys valves and seats, and a filter costs less than one seal kit.

Filters are listed by thread and pressure rating; put a low pressure one on the inlet and a high pressure one after the pump only if the listing is rated for it. Pivot couplers take the twist out of the hose at the gun and make a long day easier on the wrist. Foot valves belong on any machine drawing from a tank or drum, so the pump keeps its prime. Check every thread before ordering - a mismatched adaptor is a leak waiting to happen.