My favorite way is:
1. Put the cursor before the s-expression
2. Invoke C-M-k (kill-sexp)
3. Immediately C-y (yank) to put the s-expression back in its place, OR If I'm copying from an "untouched" source buffer, I'll use C-x u (undo) instead, to keep the buffer in its "untouched" state.
4. Paste to wherever you need this s-expression. Remember that if you subsequently copy something else in the kill ring, you can access older entries conveniently by first doing C-y (yank) and then immediately doing M-y (yank-pop). This is very useful.
Hope this helps!