Add a Null (Layer>New>Null Object). Put Effect>Expression Control>Slider Control on the Null.
Add you wiggle expression to the camera's position (Copy the expression, Alt-click the Position Stopwatch, Paste, click outside the box). Typical wiggle expression follows:
wiggle (5,50)
Then highlight the 5 (frequency) or the 15 (amplitude) and pickwhip to the slider on the NULL to get an expression like this:
w = wiggle(thisComp.layer("Null 1").effect("Slider Control")("Slider"),15)
Now the slider controls the frequency so you can keyframe from zero to whatever you want.
For a camera shake you might want just wiggle the x and y, the original expression would look like this:
w = wiggle(5,15);
[w[0],w[1], value[2]]
This modified from Dan page here:
http://www.motionscript.com/design-guide/wiggle-1-dimension.html
and linked to the slider, this:
w = wiggle(thisComp.layer("Null 1").effect("Slider Control")("Slider"),15);
[w[0],w[1], value[2]]
To put a slide on the amplitude of how big the shake/wiggle is just highlight the 15 and pickwhip to the slider. That's probably what you want.
The pickwhip is that middle symbol the look like a "@" under the now red expression controlled values.
Also, wiggling the camera's Z rotation a bit can give it a nice (and queasy) feel.
