Hi,
Using a breast pump provides little stimulation to the breasts to maintain supply long term, try expressing at least every 3 hours, and hand expressing at the beginning and end, but remember, breasts are not like sponges- it is not how hard you squeeze! Short quick squeezes, with your fingers right back from the nipple are the way to go.
Skin to skin contact will help release oxytocin and prolactin (good breastfeeding hormones) as will thinking of your baby when you are expressing, or looking at a photo of them.
happy expressing!