I know you already know, but I auto-delete my extra shards. I started with a 12 slot bag (the one from lock quests) and just picked up the items in the 11th and 12th slot from the bag and deleted them in my /shoot, hard DPS and easy DPS sequences. Something like:
Code:
Other Part of Macro
More Macro parts
/script PickupContainerItem(bag,slot); DeleteCursorItem();
/script PickupContainerItem(bag,slot); DeleteCursorItem();
(put in bag 1-4 and slot (last 2 of bag), like 12 slot bag, put in 11 and 12, 18 slot bag, put in 17 and 18)

That way you can spam drain soul all you want and you don't end up with 36 shards (Blizzard's hard limit on shards).
Hope that helps.