Pumping sucks. The machine itself appears in direct violation of the Geneva Convention. Once you actually shove your boobs into the crazy holders and it starts yanking away on your nipples, you will have the urge to flee and/or weep. Why are you even in the situation of strapping yourself into this thing? Usually something stressful, like being separated from your baby or having major supply issue related to one or more breastfeeding problems. This can make the pump look and sound even more like Darth Vader.
A good breastpump is, however, the only effective way to release and capture breastmilk without a baby. The function of the pump is actually not as much to suck as to mimic suckling. This stimulates signaling hormones (released by your brain in response to your breasts) that tell your breasts to release/ eject milk (more info).
Ideally, you would never need to pump. There are two reasons you might: 1. your baby is sick or having a hard time effectively nursing (and therefore losing or not gaining weight) or 2. you have to go somewhere like back to work or just out for the day and you cannot bring your baby with you.
One of these things is probably going to come up at some point (hopefully only the latter and not for awhile) - be prepared! Buy a high quality pump before you give birth and have a lactation consultant or a pumping expert you know (whose cool with seeing your boobs) demonstrate (on you) exactly what to put where and how to get it whirring away.
Suction, fit, posture, and practice all come into play. Only a little milk will dribble out based on the machine's pull. Remember, the machine does not really pull the milk out, it stimulates your brain to tell your breasts to spray the milk out. Only then, 3-5 minutes after you started (8-10 min. sometimes) will you feel the tingle/burn of let down and the majority of the milk you'll pump in that session all come flowing out in 1-2 minutes. Don't expect more than 2-3 oz total every 3-4 hrs.
I should also mention you are likely to find yourself with at least one broken or missing part, rendering the breastpump useless, when you need it most - say, 10 minutes before you give talk for 50+ people about chromatin remodeling proteins. Did I mention pumping sucks? Except when it's the difference between being able to breastfeed or not. Then, it just kind of suckles.