Polishing/rubbing compound with elbow grease and/or good buffer followed by wax and elbow grease again.
Maybe there are some MIRICLE products that require no sweat, just wipe on and it looks like new forever.....good luck on that one, modern science is working on it but I don't know much about them.
Penetrol by Flood is a 'paint conditioner' that is very often used to wipe down fiberglass and restore the NEW LOOK but it does not last. It has been said that it is a used boat salesman's trick to wipe down with penetrol and sell it before the shine fades. It does work well and can be redone over and over. My wife is very fussy and she uses it on her Grady White 206 which is no beater, she bought it new in 1996 and keeps it under a cover when not out cruising.