Formula for concatenating multiple fields with "commas" to seperate values

Gurus,

I could use some help figuring out how to write a formula for this problem:

I have 6 separate currency fields to concatenate in one field.  I have to eliminate values of 0.  

I wrote a formula like this:
if isnull({field 1}) or {field 1} <= 0 then "" else Cstr({field 1}, 0, "") & "% Fiber"

I can't figure out how to include the commas (,) to separate more than one value or eliminate the comma if there's only one value.

The result would display something like this:
10% Fiber, 5% Protein

...or in cases where there's only one value:
10% Fiber


Can you help?

Thanks in advance!!

Steve