Hi babi77, i'm a working mom too. the trick i think is basically to pump more frequently, which is tough in an office situation. but sticking to a schedule works. it's very true that the more you pump, the more you get. you might get less each time with less hours in between, but as your breast is stimulated more, the total output/day is definitely more.
with working, i try to structure the day so that i pumped 2X before i leave home for work, so one early morning, 5ish and then another just before i leave at 8am, 1-2 times at work, and then 2 times at home. i'm weaning her off breastmilk now so i do less now.
also, be sure to empty out your breast, i pumped until the breast is drained (to get more milk), if you don't want to increase supply, you can just pump until you get enough. At work with the manual pump, switching between the two breast helps too, let's say i start on the right breast, i pump until it slows down, then go to the left and then back to the right, so on and so forth for a couple of times until nothing comes out, i think that helps with really emptying the breast at each pump.
i use a combination of electric (double pump) at home and Avent Isis manual at work. for me, the Isis works just as well if not better than the electric, so i think it differs from person to person, you kind of have to 'mesh' with your pump.
fyi, if you want to consider the electric pump, i rented mine from Annerley Midwives. so you can 'try it out' without investing a bunch into it.