First, spend as much time as you can nursing and having skin to skin contact with baby when you're home. Co-sleeping may be a good option for you so you can nurse frequently at night and still sleep.
Second, make sure you are eating and drinking enough. I know when I worked full time I often forgot to have a drink or a snack during the day and you really need to while you're breastfeeding. Even now if I start to get dehydrated my supply tanks.
Third, consider taking fenugreek or drinking "mothers milk" tea. That can boost your supply.
Fourth, try to incorporate lactation supporting foods into your regular diet. Oats are a good one, there are others. I eat granola or oatmeal for breakfast almost every day and that helps keep my supply up.
Fifth, you may want to look into supplementing with donated breast milk. Either to use for now while you're working on building up your supply, or if for some reason these tips don't help. (I think they will)
Lastly, make sure you check your pump regularly to make sure the valves are working properly and not worn out. When they start to get worn they don't extract milk as efficiently and you'll get less even if there's plenty in your breast.
I pumped very frequently, even if it was only a little bit, lots of frequent gentle stimulation increased my milk supply. I didn't wait til I felt I needed to pump or could produce "enough to make it worthwhile." Probably every 1-2 hours if I recall.
I had a similar problem. I was trying to build up a milk stash so I could go back to work and as the days went by, I got less and less when I pumped. My supply was a little high before I started pumping and by the time I stopped my supply had taken a very noticeable hit. My baby was really unhappy.
I didn't have the same problem with the electric pump I used with my first, but it was a double as well and I've read that pumping both breasts at the same time encourages more milk flow. I don't know if that has anything to do with it, but I'm just putting that out there.
I tried taking fenugreek (which has always worked well for me) for a while then going back to pumping with the manual pump. The fenugreek helped, but my supply still dropped, just at a slower pace.
I wish I could say that I got another electric pump and all was well, but I couldn't afford it at the time, so I gave up on the idea of going back to work. I just focused on getting my supply back up. The lowered supply really scared me.