use a mild soap/cleaner and soft brissle brush to clean off any dirt, mold etc then treat it with a protectant/conditioner designed for your type of roof. while your up there check all your seams, vents etc for any cracking sealant and re-apply as necessary to ensure a water tight roof.
*Currently trailer-less but shopping
07' GMC Envoy w/ P2 controller (current TV)
88' Chevy K5 Blazer (soon-to-be TV)
74' VW Superbeetle (1303)