do you live in an area with a lot of humidity? i know that mexico has a lot of very dry areas, but i know there are some more swampy spots as well, and rainforest-type spots.
my experience tells me that guitars left sitting out have oxidized strings much more quickly than ones sitting in cases. how is the room, humiditywise? high humidity? loose water particles often? i don't seem to have the trouble some of my friends do, and my guitars tend to stay in-case.
my solutions tend to involve
GHS fast-fret, as mentioned earlier (a kind of mild wax i use both all over the fretboard when changing strings and on strings after playing), and when a fretboard is really gunked-up, taking a mild steel wool and cleaning the board (first rubbing crosswise, in the directions the frets point, then nut-to-bridge to match the grain of the board), and when my fretboard starts getting light-colored/dry, i like olive oil for treating it (a cheap extra-virgin olive oil works well, as they have little scent to them on the whole).
using fast fret regularly, i almost never have to change strings, though i generally prefer to do so once a string breaks, as i only break one every few months or so, usually during tuning, since i'm not much of a string-bender (i bend strings often, just not wildly as a great blues man might)