Stopping/Reducing Wear On Keycap Letters...
This might seem kind of obvious, but I'll share my experiences just the same.
Over the years I've had several "cheapie" (yet still highly
reliable) keyboards that the key legends on keycaps under my left hand started showing very noticeable wear in less than a year of use. The legends on these keycaps were the "painted" on type.
It's probably due to the unique way I hold my hand while typing and the fact that keyboard is used mostly for word-processing...other folks may see wear on different keys.
I stopped the wear dead in its tracks on all the affected keycaps by simply covering the ASDEC letters with an approx 7mm X 7mm square of CLEAR
'cellophane' tape (aka "Scotch"(TM) tape).
It's absolutely vital to use fresh and quality tape and it must
be the clear tape...not the 'frosted' so-called 'invisible' type. It's also absolutely essential that the tape be no closer than about 0.5mm from any edge of the keytop, and of course the keycap must be clean of any contaminants before applying the tape. I cut the squares of tape using a virgin X-acto blade and applied the tape to the keycaps using tweezers to make sure no oils were transfered from my hands onto the tape, and to reduce the chance of trapping bubbles under the tape. I then 'buffed' the tape into the keycap. (The reason for using an X-acto blade (or equiv) is that cutting with a single blade the tape from the non-adhesive side puts a properly oriented bevel on the edges of the small square of tape...and those bevels help keep your fingers from 'catching' the edges of the tape as a fingertip wipes across a keycap during use. Using scissors doesn't create that bevel thus the tape would be more likely to 'fray' around the edges over time and would more easily detach from the keycap.)
One keyboard I've been using frequently for over 5 years has zero additional wear on those keycaps, and I haven't yet had to replace the tape on any keycaps on that keyboard (once I discovered to leave that >= 0.5mm gap between the edges of the tape and the keycaps, and
to not use a 'bargain' brand of tape).
also be possible to use clear
nail polish or a spray-on clear coating product to protect the legends.
I'd test it first on an unimportant keycap, as the chemical solvent (e.g. acetone) used in some polishes may
slightly "melt" the keycaps and/or the legends when first applied (I don't know if nail polish still contains such solvents or not, but if they do I'd bet keycaps made from cheaper plastics can be affected by it). I'd also bet that using nail polish will likely require re-applying it every now and then. If
one has a problem with any tape fix, perhaps a coating of clear nail polish or spray-on coating over the tape would help ensure the edges of the tape couldn't come up or the tape shift over time due to adhesive failure in some conditions.
Instead of tape or nail polish, I've also successfully used Krylon clear spray coating on some electronic assemblies (keycaps have to be removed from any switches first). Several light layers sprayed on over the lettering worked better than one thick coating. Warning! Spray-on coatings also often contain solvents, so one should pre-test and great care should be used while applying the coating to ensure it won't 'melt' any plastics or the lettering. A can of spray-on coating is also more expensive than a small bottle of cheap nail-polish, but if carefully applied, the spray-on coating results can be far superior.
Application note...it's possible that on some light-colored keycaps that over time there might be a noticeable outline around the tape (build-up of dust around the edge of the protective tape or labelmaker tape). This also might be eliminated by coating over the tape and the entire top of each respective keycap with clear nail polish or a product like Krylon clear protective coating.
Another application note...none of the keyboard keycaps I tried this on have the deeply sculpted "spherical" keytops found on some mostly older keyboards. The 'tape trick' might not work as well for those kinds of keycaps (see http://www.overclock.net/t/491752/mechanical-keyboard-guide#post_6009683
for more info on keytop shapes and other keycap information).
Keyboard Keycap Lettering Restoration...
If you already have keycaps with significantly worn-off legends, you might try using 'dry-transer' (aka 'rub-on') letters/numbers...or maybe even printed-out letters from Dymo, Brother, etc labelmakers. The dry transfer kits are usually available at office supply and hobby stores, they aren't that expensive, and they come in different sizes and many colors (note: that might be of particular interest to gamers who might want to customize certain keys with letters that stand-out from the surrounding keys.) However, some
'peel-off' lettersets use a relatively thick vinyl material which can be much too thick for this use. You would be able to feel the letters on the keys, and it may not be acceptable to you. But 'rub-on' dry-transfer letters are extremely thin, being more like an ink/coating than a thick vinyl film you apply on the keycap.
After applying the new letters on a keycap, I'd try protecting them with either the 'Scotch'(TM) tape, clear nail polish, or a spray-on clear coating as I described above. All warnings still apply.
I haven't tried that on actual computer keyboard keys, but I've had excellent
success doing this on various regular 'on/off' electronic switch keycaps, project cases, etc.