What is the best way to cover your camera and microphone?
For the camera:

You could try a small, completely opaque vinyl decal (or just a piece of one cut to the appropriate size, like those window decorations some people use that cling to glass without leaving a residue). I believe most of them adhere using a combination of static cling and a tacky (but not sticky) surface.

For the microphone:

As for the microphone, I don't know of anything that adheres to the brushed aluminum surface of a Mac that doesn't act like a sticker or tape, which may leave a residue behind. Magnets are a bad idea because they can damage internal components over time, like the speakers. Simply covering the microphone isn't a good option though because of how/where Apple installs the microphones in Macs. Sounds can still get through, although it would be muffled. 

Instead, it's been suggested that buying a TRRS 'dummy jack' and inserting it into the headphone port is more effective because it mutes the built-in microphone until you unplug it. I personally haven't been able to locate these 'dummy jacks' that people speak of, but instead a headphone splitter with Mic support should do the trick and doesn't have to be too costly.

If you can't find a TRRS 'dummy jack' (one that has the mic support) or a headphone/microphone splitter you could try purchasing a pair of cheap headphones that have a built-in Mic (maybe even from the dollar store?) and cut off the cord, leaving just the little end that plugs into the headphone port. Note: I have not tried the cut-off headphones trick myself, so I can't guarantee that it will work with Macs.

You also need to remember that if you do use one of these methods that while the Mac's audio jack/port is occupied your internal speakers will not function, as they've been switched off. This means that to listen to music or watch videos using the internal speakers you will need to remove the splitter or dummy jack.

The headphone splitters with microphone support look like this:



You'll notice that the little end has 3 rings on it - not only two - the 3rd being for microphone support.