Although I used to get very anal about keeping lenses spotless, after a few years in this job I just kind of gave up. Iraq and Afghanistan have provided some of the most challenging working environments I've ever come across...heat and dust in the summer months, freezing cold and rain/snow in winter...
Nowadays I use a cheap cotton cloth on the filters and 'proper' lens cloths and army-issue Sylvit cloths
http://www.globalmilitarysupplies.co.uk/inc/sdetail/2234 (well-washed frequently) on the front and rear elements themselves.
Small bits of dust have no noticable effect and major dust/fine-sand get blown off whenever I notice.
When travelling in the back of open vehicles in the desert I generally wrap an issued sweat-rag (cotton) or shemagh over the front element to minimise the amount of dust that settles...
For
major dust and dirt I use a 1.5" paintbrush to get big boulders off...
After each assignment, I brush off as much dust as possible then using a sweat-rag soaked in water, wash the exterior of lenses and cameras as thoroughly as possible, and a wetted toothbrush to clean off the rubber grips...
Desert dirt combined with sweat from hands and face (you tend to drip onto the eyepiece and down the camera-back when shooting), turns into a kind of concrete if left too long...