No kids (yet), but I do have a couple of suggestions since I fly a few times a month and usually try to help out when parents bring small children - I figure playing with the kid is a lot more fun than hearing them scream :-)
Obviously, since you are asking this question, you wont be one of those people changing the diaper on the tray table and ignoring the child if they scream! Reading some of people's horror stories on other sites can also give you a lesson in "what not to do"!
1) Don't take noisy video games - hearing those noises the entire flight will not win you points with neighbors.
2) If you are getting the stink-eye from others (mentioned in another answer), yes, realize that they are seeing your child in a very stressful situation and apologize. YMMV, but many people will either settle back down or stop and you might be surprised to get some help!