For small openings, I use the red stuff that comes in a tube for a caulking gun (and no, I'm not referring to any type of silicone caulk!).
For larger openings, I find myself doing some sheetrock work. If there is a rocker on site, I'll hire him.
While I'm at it, I will often install a stub of pipe (1" or larger) to allow running data, etc., in the future. This pipe is plugged with the red stuff.