Assuming the milk has come in already (See the Inducing Lactation Resources page if not), foremilk comes out first; hindmilk comes out second. The hindmilk conatins milk fat which separates when expressed. There are specific pumping routines that can increase hindmilk or foremilk production. Hindmilk is much harder to create and too much hindmilk production can lead to severe clogs, milk blisters, and mastitis (warning:NSFW). However, hindmilk is much sweeter; for women that do sell their breastmilk, a higher fat content is very desirable.
To increase hindmilk production, the most important thing is letdown. Letdown triggers hindmilk to increase. If you are pumping every two hours, the trick is to pump until empty, and pump for an extra 10 or 15 minutes, until you see or feel another letdown. Then every 30 minutes between pumping, trigger letdown by stimulating the nipples. Do constant massages throughout the day. When you trigger a letdown, just replace the nursing pads onto the breasts, put the bra back on, and let the hindmilk build up. You're signaling to your body that it needs to make more hindmilk.
To increase foremilk production, make sure that when you're pumping that you don't have another letdown. Try to stick to one letdown. Drink even more water and keep your fat really low in your diet. Don't trigger letdown between pumping. And when you pump, just pump until you know that you have only several drops left. That signals to your body that it needs to make less hindmilk.
See the Breast Pump Resources page for more information on breast pumps.
See the Inducing Lactation Resources page if you're interested in starting lactation without a pregnancy, for an ANR, or for your partner.
Frequently Asked Questions
About Inducing Lactation
Can I use a TENS Unit to help induce lactation? (warning:NSFW)
What does breast milk look like? What does breast milk taste like?
Are there health concerns from breastfeeding or letting someone drink my milk?
It's perfectly fine to try another person's breast milk if you know their health history. Breast milk is a body fluid and therefore it's possible to transmit pathogens. The next biggest concern is Mastitis (warning:NSFW) which can be lessened with Lecithin and a proper latch technique. The Piercings and Lactation page (warning:NSFW) addresses this concern as well.