Don't overstay your welcome. New parents are sleep-deprived and every opportunity to nap needs to be taken advantage of. Keep visits short, 1-2 hours at the most.
New parents: don't be afraid to let your guests know "Hey, it's 8:30 pm we're tired and are going to hit the hay, so your going to have to hit the road," or something to that effect.
Come bearing gifts. Bring a bag of diapers or better yet, food. New parents rarely have time to feed themselves let alone cook. Never expect or ask new parents to pick some food up for you all. Again, time for sleep = important.
Lend a hand. If you don't bring something with you then offer to do a chore. In addition to having little time to cook, they also have little time to clean. So, don't be surprised when you visit and the house looks like a tornado rolled through. If it doesn't, then they obviously went to a lot of trouble to skip naps to make it look presentable for you.
Mini-babysit. If possible, offer to bottle feed the baby while they sleep. You get to enjoy the one-on-one time with the little one while they catch up on some z's.
Don't expect much. Until the child is at least 3 months old, don't expect new parents to hang out long, be on time, or have anything but tentative plans. New babies require a lot of flexibility, time, and energy.