I have a few questions? what brand of bike is this and what lever components? is this a commuter, a weekend tear up the trails MTB or a bike that sees seldom use?
I do agree with above though that keeping your cables clean is very important.
for a lube/cleaner for the shifters I recomend Triflow or some other light oil based spray with teflon. WD-40 while good for getting things moving is not a good long term lube.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk