I have a pair of Gore-Tex walking boots that I have used a fair bit now and want to make sure I keep them serviceable for as long as possible. What is the best way to maintain Gore-Tex walking boots? Do they need reproofing occasionally or is just brushing the mud off enough?
The Gore-Tex trail shoes I'm wearing right now have the Gore-Tex layer on the inside (between my socks and the outer suede/synthetic). I'm not sure that you can do anything to fix that layer once it is worn out.
I would make sure you clean mud off your boots as soon as you can (usually once you get home from a hiking trip).
If you have leather boots, then some rub in some dubbin to stop them cracking and keep them waterproof.
If you have suede/synthetic, then you should be okay with a NikWax treatment every few months (depending on how often you head out) as long as you keep them clean.
Whatever you do, the membrane will wear out in a few years. That's one of the reasons why many people prefer leather boots, because once the membrane starts to leak, they can still waterproof the leather (it's much harder to perfectly waterproof textile).
That said, you can always try to keep the membrane as long as possible. One thing manufacturers recommend is to never wear cotton socks in the boots, as cotton gets abrasive when wet and you could easily wear out the shoe inside to the point of damaging the membrane.
Another thing is to clean the insides from debris that could puncture the membrane. Apart from that, I don't think you can do much. Waterproofing the outside layer will probably keep the membrane working better (like DWR on clothes, since leather soaken with water does not let the vapours through that well), but I don't think it makes it live longer.