using label helper with HABTM checkboxes

I have a list of category checkboxes whose HTML looks like this:

<input id="categories" name="categories" type="checkbox" value="4" /> ...from... check_box_tag("categories", cat[0], @post.categories.map {|c| c.id}.include?(cat[0]))

I'd like the label to select the checkbox it's associated with... but all of the category names are "category", so when I click any of the labels they all toggle the first checkbox.

How can I code the label tag to associate the label with the checkbox?

Thanks,

SH

PS: Ideally I'd like my params[:categories] to look like [1, 3, 5, 8]