Each year during haulout give every seacock and thru-hull fitting a thorough inspection. With a nonmetallic hammer before launching, from the outside of the boat, give each thru-hull a tap. Your goal is to find out if any are loose or weak. Inspect them from inside, too.
If the seacock is properly installed, its base will be tightened down firmly against a wood pad molded onto the hull, so you won't be able to see much of the threaded thru-hull fitting.
Seacocks should be greased yearly. This can be easily done by putting a small amount of grease on the end of a wooden dowel and insert it into the thru-hull from the out side. The inside can be greased by removing the hose that attaches to the valve.
Bear in mind that the best seacock in the world won't help you if the thru-hull fitting to which it's attached fails.
Also smart boatmen carry tapered wooden plugs to fit every hole under the waterline. Drill a hole in each plug and tie it to the thru-hull it serves. Use string that is easily broken, because when the water is gushing in, you will undoubtedly discover that you have no knife handy.
Check for signs of water seepage or a damp green paste around the thru-hulls, too. If you find any, take out the fitting, rebed and reinstall it. If your boat has a cored hull, ask an expert to check for water intrusion.
As a precaution close all seavalves before leaving. This serves two purposes. It allows you to inspect each valve before leaving the boat and will prevent a sinking if a hose fails.