I'm a newbie to Fastkeys. I have been using ActiveWords and was looking for something more user friendly.
The "string" scheme that I've worked with and will continue to use, is the string followed by an x. This is something that is almost never typed in standard writing and becomes automatic (in your mind) to use.
For instance:
faex
for
Fresh Air Exhaust
So if I wanted to type FAE I could do so without fighting the auto change (yes I know I can now also change the way capital letters are responded to).