Do you use a laundromat or other public laundry facility? You might want to wait until you're the only one there for this one.
Go through the trash and pick out discarded detergent containers, both liquid and powder. Rinse a couple of "empty" liquid detergent bottles into your laundry and that should be enough. For the powder, bang the boxes on their corners a few times and pour the powder out into one box. Do it on all four corners. You'll be surprised how much is left in there.