bnrodn,
"Super Lube" silicone lubricating grease works well for urethane bushings. Do not use a multi-purpose grease - this type of grease is not silicone based and will ruin polyurethane bushings.
You can buy the silicone grease it on Amazon (and other places). Recommend that you buy a grease gun and a 14 oz. cartridge so the grease can be pumped into the channels of the bushings. Taking the sway bar off to reapply grease is a lot of work (in my opinion), a grease gun makes the job very easy and quick.
HTH.