Silica gel... it comes in little pouches and is generally pcked in with many optics to keep moisture out during shipping. If you don't have any laying around, check with your local sporting goods store... most binoculars and spotting scopes are shipped with at least one pack and they should be plenty willing to just give you one or two. If you're out in the desert where temperature swings are pretty extreme between night and day, condensation can become a big problem with any optics.


Another option is to check with your local university's chemistry department and see if they'll let you have some of whatever dessicant they're using, though you'll probably have to make your own pouch for it out of whatever breathable material you have handy.


A last resort is to grind up some instant rice, though you'll have to mix it about 50/50 with table salt to keep small organisms such as fungi and bacteria from using the rice as food and contaminating everything... a far worse fate than moisture in your equipment.