Clean fuel is vital. Every 2nd or 3rd tank allow the heater to burn completely dry until it goes out by itself. This will burn out the tar deposits in the wick top. Wick tops are fiberglass in all but the smallest and earliest models. The wicks are not consumed, don't need trimming but will clog with unburned fuel components. Water in the fuel will dissolve the adhesives used to join the wick sections and also interfere with capillary action.
Thanks for the advice on keeping the wick clean!