If you use Safari, go to Safari's Preferences, and under the Privacy tab, check "Ask websites not to track me". (This may not help much, but it can't hurt.) Also, make sure the "Block cookies" setting is set to "From third parties and advertisers" (this is the default).
If you use Safari, install the AdBlock extension. Under its set of options, click on the Filter Lists tab, and enable the EasyPrivacy list.
Under System Preferences, Security & Privacy, in the Privacy tab, under Diagnostics and Usage, make sure that "Send diagnostic & usage data tto Apple" is unchecked. (This is the default.)
Cut out a little piece from a sticky note and cover over the camera with it. This way you can uncover the webcam when you want to use it, while ensuring it is not used when you don't. (Just don't get the sticky, gluey part of the sticky-note on the camera lens: you want that to be beside or above the lens.) Yes, there's a little LED that comes on whenever the camera is active, but this way you don't have to worry about failing to notice the LED.