[postal]

clay or melted candle wax are both popular for
keeping paint or bedding out of places.
Postal

 

[792]

Tape and golf tees

 

[fullchoke]

Or not being a golfer, I just finished using tape and broken cotton swabs.

 

[Fumbler]

I like hot glue for little holes. Fill the holes with hot glue then take a knife and just trimm it flush with the hole. After you paint it, just push the glue through to the other side.
Otherwise I use all sorts of tape. For something like the ejection port or scope bells I take electrical tape and put half of it on the inside surface of the hole. Take paper and stick it on the other surface of the hole to close it all up.