What is the best way to keep urethane bushings on my sway bars from squeaking?

I got the Energy Suspension version and they came with some teflon type grease. There are some versions that have a nipple on them for easy lube application. Otherwise you probably have to remove them, clean them, reapply grease and reinstall.

What happens if I don`t do anything, just let `em squeak?
Will they wear out fast?
Would spraying them with silicone help?

I wouldn't think they would wear out fast, but the noise may be annoying and it means the anti-sway bar may be binding to some small degree. On my 1986 Fox body, I drove up on my ramps to elevate the front of the car, loosened up the endlinks of the anti-sway bar a little, then unbolted the bracket that holds the bushing. The bushing has a cut in it so it is easily removable. Wear eye protection of course because of the rust, sand, oil, fluids and dirt, etc.

I would not expect spraying will not penetrate into contact areas between bushing and sway bar.

Also you can use locksmiths powdered graphite