Many women have a hard time keeping their supply up when they are pumping versus nursing, since the sight, smells, and sounds of your baby are what help let down your milk. Try to stay as relaxed as possible. Massage your breasts before starting, and look at a picture of your baby or watch a video of her on your phone while you pump. A blanket or onesie that smells like her can also help stimulate your milk production and make pumping a little easier. Always make sure to drink plenty of water.
Pump as often as you can. A good rule of thumb to follow is to do it whenever your baby normally eats. If there are days when you don't have time to get in your full sessions, try to pump for at least a few minutes. Skipping a session will not only make your milk supply dwindle, it can also make you engorged and put you at risk for mastitis.
If you find your production is low, consider talking to your doctor about taking herbal supplements that may help.