I only pump until i get no more milk as well - after doing breast compressions...
My situation is different since i only pump once a day - so it's for 20-25 minutes during my 6 hr work day and i get my full 12 oz all in one sitting... (which is a lot for a baby to take - he sttn about 9 hrs, so his daytime intake is quite high)
You can do breast compressions to up your output at each sitting, but i don't think that you are having a low amount. 12-16 oz a day? sounds about right to me, depending on how long you are gone.
Instructions on compressions:
http://www.kellymom.com/newman/15breast_compression.html