Free: Contests & Raffles.
I use lens cleaning tissues that I buy at Costco. Like these:http://www.costco.com/Browse/Product.aspx?Prodid=11532286But then I don't really have expensive optics, so my opinion really isn't worth much.
I took 4 of the Swarovski North America reps hunting a few years back. They said the worst thing is a lens pen as it keeps grinding in the dust you wipe off. They advised soapy water and dry lightly with a very soft clean lens cloth. Only clean when absolutely needed. I only clean about 2 or 3 times a year even though I spend several months in the field.
I use lens-pen. Has a built in brush for big debris and the cleaning tip works really well. They are like $10 just about anywhere cameras are sold...Sent from my sniper hide...