Boots, like any other part of your diving or snorkelling gear, need to be properly cared for and maintained, which means doing the right checks before and after use, cleaning them properly and storing them correctly. This will guarantee they last you a long time.
After use rinse the boots in fresh water, this is particularly important after contact with salt water. Salt water can cause neoprene to lose its flexibility and can create nasty smells if unwashed.
It may be best to just place them in a tank of fresh water to soak for a short period of time (15 minutes or so). Most popular snorkelling & diving destinations provide tanks for your individual equipment. You can also use a wetsuit shampoo on your dive boots, baby shampoo works just as well. Do this every once in a while and rinse with fresh water to ensure your dive boots last as long as possible.
Like all other equipment, you should check your dive boots regularly to ensure they are fit for their purpose. You are looking for any tears, splits or fraying. These, when small, are usually very easy to repair with diving tape or glue. If there is a large tear however, you will need to replace your dive boots.
Store your dive boots in a cool, dry place, away from direct sunlight. You do not want to degrade the neoprene material which is why you should also store boots, or any other neoprene equipments, away from car exhaust fumes, so not in the garage, and avoid spraying aerosols near them.
Do not bend or fold your dive boots for compact storage, this can have a detrimental effect on the insulation efficiency of the boots.