Buy a camera lens cleaning kit from a place like Adorama or B&H Photo. A blower to remove dust, then a lens brush to further clean, followed by a lens pen that finishes the proccess. Depending on how grimy they are, you may need lens cleaning fluid applied with a microfiber cloth to remove heavy grime before the final cleaning with the lens pen. Same system I use on my $4000.00 camera lenses, and it works well. Last thing you want to do is just rub them with a paper towel, as you risk damaging the coatings and scratching the glass.
Larry