If you want to make your speargun last, then you need to know how to clean it correctly. Spearguns rust very quickly if they are not washed down after every dive, and washed correctly. I usually just give mine a hose off, but I ensure that I pull the spear out, move the trigger around and get rid of any salt water that I can. Sure, the ends of the spear will rust anyway, but if you can minimize the rust then you will be a long way ahead!
Generally the trigger mechanism is the item that stops working first, because of a buildup of junk and rust that occurs. If you are finding the trigger and safety catch is getting harder to use you can usually pull it all apart and give it a good clean. A bit of CRC goes a long way into these. This is especially the case if you are doing a lot of diving from the beach. Sand seems to find its way into the gun even when you don’t throw the gun in the sand!
Whilst you are cleaning the gun you should be checking that everything is ok. Pull the rubbers from a few different angles to check how much they are worn, and have a close look at the bungee cord attaching the gun to the line. Your mono line is also something which breaks fairly regularly, so check that it is in good condition. All you need is one sharp rock to damage the line and you risk losing both a good fish and your spear if this is the case! The most important thing is to keep your gun looking like new. Avoid chemicals on the gun body and on the rubber, and make sure you clean out any sand and salt water from the guns mechanisms.