I've always heard that bandaids are the way to go - electrical tape can gunk up your webcam when you actually want to use it.

I have had some success with aluminum foil (the kind you might want to use in a kitchen) -- place that over the webcam lense and then use whateever tape you want to use to hold it in place.

I have my camera set to take a picture every 20 minutes, but when I need privacy I just stick a post-it over the thing, and fold it down.