I'm totally going to get panned for sharing a tip from Ken Rockwell... but here goes:
In response to PeterJ and V8Beast and those with cameras without auto updating custom functions (e.g., 7D)http://www.kenrockwell.com/canon/7d/users-guide/
To Make a Longer Change to a Stored Setting
The 7D system allows all sorts of ways to make things easier.
As we just learned, if you change something in a C mode, after the camera resets in usually about a minute, all your temporary settings are reset.
What if you want to keep using slightly different settings for a longer period of time?
If you want to save your temporary settings for up to about a half-hour or so, while in a C setting, press:
MENU > WRENCH • > Auto Power Off > (select longer time period) > SET.
Now the 7D will work in your C mode in your new settings for that period of time after you make your last shot, and then after that time (30 minutes maximum), automatically reverts to whatever you saved in that C setting. (If you leave that C setting and return, it also resets instantly.)
Here's the clever part: you keep that setting for 30 minutes, and when the 7D resets to your saved C setting, the time-out period also reverts to whatever you saved before, probably one minute! Thus you can alter a C setting for a while, it won't change, and when you're done, Voilà!, the 7D gets you back to where you wanted to be automatically!