Here's the deal with mouthguards: they wear out. When used as directed, they are in direct contact with two things basically designed to break stuff down: your teeth and your saliva.
Wear
To ensure its normal lifespan, first of all, don't chew on your mouthguard and don't stick it in the holes of your helmet, say, during water breaks. One of the advantages of a custom mouthguard, actually, is that it stays in your mouth all practice since you can talk and drink with it in and it gets less manhandled.
Wash
After every practice, give your mouthguard a quick rinse under cool water. That's all it needs on a frequent basis, but you may wash it now and again with mild soap or effervescent denture cleaner. Your favorite toothbrush and toothpaste are probably too abrasive for your mouthguard, which is a lot softer than your teeth. Hot water can soften and distort your mouthguard, and leaving your mouthguard in the sun is also not a good idea for the same reason.
Store
Store your mouthguard in a hard, ventilated case so that it doesn't get crushed and dries quickly after it's rinsed.
Replace
Even with proper care, the life of a mouthguard is fairly short. The dental coverage that you get with Shock Doctor mouthguards, for example, is good for one year. Dentists typically give a two-year lifespan to custom mouthguards, which should be inspected every year for fit and wear.
