G'Day Steve,
There's probably lots of ways to do this...
My suggestion is to use regression in the data analysis add-in. Amongst all the other stuff this produces, the "intercept" coefficient (b) and the "X variable 1" coefficient (m) should give you the centre trend line:
y = mx + b
To normalise your data to oscillate around a horizontal line, calculate y from the the above equation in a column beside your raw data, and produce your normalised data by subtracting this from your raw data.
Clear as mud?
Al.