I have been using these gaiters for a couple of years now and they are very good at keeping stuff out of my trainers. http://www.inov-8.com/New/Global/Product-View-DebrisGaiter-32.html
Their weak point however is the flimsy rubber under-arch string. Also the faff in putting them on (they have to be put on the ankle before the trainer) and getting the rubber string in the right position.
One of the strings broke on this year's 100 and the second broke on last week's WoW.
They had come with no instructions and I'd thought that the rubber strings were pre-sewn into the gaiters.
Maybe other users of these gaiters know this secret, but I have only just discovered it: The two tabs which hold the rubber string in place each have velco on the inside! This means that I can now replace those strings; also in future I can put on the gaiters then add the rubber strings.
I hope that somebody else finds this useful.