Sub totals are the same as totals, just check the following:
add the order by corresponding to your list - usually add the item to the order by list
ORDER BY some_name_wharever, my_subtotal_thing_to_be_ordered
create a new group using the wizard, right click the root of the report on the report inspector and select "add report group"
- create a new variable just like the total one but mind the reset and increment group
That should do it. Hope it helps. Good luck.