It isn't possible to put REMs inside @-loops, and therefore I haven't commented these before.
I just discovered that you can put strings that don't belong to a variable, anywhere within the loop.
I've now started commenting like this (ignore the bad example):
@For( i:= 1 ; i <= 5 ; i := i+1 ;
"## If counter is above one, add comma before next number ##";
str := str + @If( i > 1 ; ", " ; "" ) + @Text(i)
Result: 1, 2, 3, 4, 5
The ##'s to make it easier to distinguish comments from code.
This is a basic knowledge kind of tip, so I tag it with Tims proposed tag.