Triggering letdown while pumping

jchasmc09

I'm sorry if this has been asked before.  

I currently have an Ameda Purely Yours.  I am trying to build up a stash of milk for when I return to work.  I am having a hard time getting a letdown while pumping.  When I BF DD, I can hear (and feel) the letdown, but it doesn't usually happen while pumping, so I usually just get what the pump can squeeze out without it.  (Usually ~2-3 oz from both sides.)  Is there another way to encourage the letdown?  I start shallow and fast on my pump, but it doesn't seem to help much.  TIA!