This is the first that popped into my mind.
If you want invincibility and the likes, do this:
Find your sp config (config.cfg), usually located at "C:\Program Files\Steam\SteamApps\common\call of duty modern warfare 2\players". Open it with notepad, wordpad or any other text editor and add to the beginning just under "unbindall"
set developer "1"
set sv_cheats "1"
set thereisacow "1337"
bind KP_ENTER "toggle cg_fov 65 80"
bind KP_MINUS "player_sustainammo 1"
bind KP_PLUS "god"
After editing the file and saving, set it as read only as the game will over write the changes. Of course you can set the key binds to anything you wish. The only thing is, you may have to press the keys at the beginning of each level start and this works for both sp and co-op solo missions.