At the hospital bf class they said to resist the urge to pump extra when your milk first comes in because you'll constantly have an over-supply and you want to match with baby's needs. But, I've heard from friends when your supply is matched to your LO's needs, it's hard to pump any extra for a freezer stash for when you return to work. For example, if you pump while DH feeds one of your previously pumped bottles you seem to not be able to get more than one bottle ahead or so (which isn't enough to last a whole day). So, how did you build your stash??
Re: How did you build up your bm stash?
I pump one side (the side I don't feed on) first thing in the morning, right after her first feeding and get at least one bottle's worth - for the moment -right there (3oz-5oz). I also pump both sides at night just before bed and get at least a bottle's worth. We only give one bottle a day so I am gaining 3 oz a day typically for my freezer stash.
This works well for me because I don't have to worry about pumping after every other feed during the day when I'm solo with the kid and hands free time to pump isn't easy to come by.
Later, around 5 weeks (which was 2 weeks ago). I had to use up my initial stash with the snowstorm when our power went out, so I just started building it up again a few days ago. I was lazy a few days and didn't pump, but I have about 15-20 oz in the freezer at the moment.
TTC #1 Cycle 14 - IUI#1=BFN, IUI#2=BFP | TTC #2 Cycle 8=BFP!!

Just a little side suggestion about your stash that someone told me. When I went back to work, whatever I pumped monday went to daycare on tuesday, and so on for the rest of the week. On Fridays whatever I pumped I would freeze, then I would use some of my frozen stash for Monday bottles. It was a great way to get my stash used before it wasn't usable anymore. To be honest, I never really needed my frozen BM, but I am still glad I was better safe than sorry. Towards 9 months I started pumping less at work because I had the stash to use up. Also, remember you can pump on the weekends to add more as well!