Adequate anode protection is so important to the integrity of your boat that care should be taken when scheduling anode replacement. When an anode appears to be half its original size, the anode is nearing the end of its life because its electrical connection often degrades so much that it may no longer provide adequate protection. Even if they still look good, it is recommended to replace anodes every year.
Installing an anode ensures a good connection to the metal it is protecting. A clear sign of a bad connection is an anode that has been in the water for several months and still looks new, so make sure there is nothing between the anode and the metal it is supposed to protect. The anodes should dissolve, and if they don't corrode, they are not working.
Also, never paint anodes that mix dissimilar metals; only the most active ones will work properly. Protect trim tabs separately; and don't glue them to other parts of the boat. It is also a good idea to always use new fasteners when installing aluminum sacrificial anodes.