Decimation Sections

So I have some clothes I need to decimate before export. For example, one of them's a jacket. I can nicely get it pretty low, but the zip gets messsssed up at the low percentage.
How can I make the zip (or any part) a separate section from other parts of the jacket so that I can weight the decimation and reduce the problematic parts less than I reduce the rest?
Comments
Does the zip have its own surface? if so you should see it in the Weights tab of the Decimator pane, after clickingPrepare to decimate, with 1 next to it - those numbers are the weights, click to change them and then, I think, click Set Weights.
Ah, that's right, they already existed. I didn't see that there was a different tab for weights. Thought they'd show up in the tree view of the main tab. Thanks a lot.