Assuming you've found and fixed all the leaks...
Clean and then dry out the bilges...really CLEAN 'em, with lots of detergent and water, followed by thoroughly rinsing out ALL the dirty water. A power washer will allow you to get into the areas that are inaccessible to clean by hand.Once you've gotten the bilge really clean, use hand pumps, sponges, turkey baster--whatever it takes to remove all the water the bilge pumps leave behind. Then leave hatches open for at least a day so that plenty of fresh air can circulate to finish drying out the bilges...even turn some fans on to accelerate drying.If your fridge/icebox, shower or heat/ac condensate drain into the bilge, install a sump and reroute those drains into it...sumps are a lot easier to keep clean than whole bilges. Humidity, condensation, runback from bilge pumps, etc will almost always cause some areas of the bilges to be damp, if not actually wet...so unless you want a boat that always smells a bit funky, you need to manually remove as much water as possible BEFORE it can sit and stew in the summer heat...and there's no way to avoid giving the bilge a real cleaning at least once a year...'cuz jump dumping in more bilge cleaner and/or bleach and letting the bilge pumps do the work for you only knocks down odor and leaves you with a soapy smelly bilge...you have to flush all the dirty water out too. You wouldn't just drain a kitchen sink full of dirty soapy dishwater and end up with a clean sink...you have to rinse it out. I dunno why so many boat owners think they can have a clean bilge by just draining it without rinsing it out too.