Diaper Bag Essentials

Packing a diaper bag... This is something that actually varies from person to person. Every situation is different. If you take a bus or don't own your own vehicle than you might want to pack a magazine and some more toys. In the summer you wont need as many blankets as the winter. Whatever your scenario, here are some things to consider putting in your diaper bag:

Diapers - They say for newborns about 1 every hour you are gone. I just put however many I can fit after everything else. That way I never run out.
Wipes - I suggest getting a travel case. They are small, cheap, cute and you really don't need a whole wipes container, anyways. My first baby I carried the whole big container in my diaper bag and it took up too much space. 
Diaper disposable bag - These are really small bags, you can get a little box of them at the dollar store. I love them. They are really helpful when you are at someones house and need to throw out a stinky diaper. 
Diaper cream - This is a must when you have a newborn. Their poor little bottoms can break out so easily. When Isabella was only a few days old her bum broke out in a rash so bad it was bleeding! Apparently this is normal and just needed some kind of butt cream prescription but for a new mom I was freaking out! 
Bottles - I would always pack a bottle in one side pocket and a bottle of water in the other. This way I was prepared to make a bottle no matter where I was.
Formula or breast pump- This is easier with the smaller cans of formula.
Hats - Not so much in the summer but packing a few hats is good for any other season where it can get a little chilly. 
Socks - 2-3 pairs. They can double as mittens. 
Pants/shirts/onesies/sleepers - However many you feel fit for the situation.
Sweater - I always bring a sweater. I would also recommend bringing one for you too, just in case. Even summer can have chilly nights.
Receiving blankets - About 2. They come in handy for many thing. Same with burp cloths.

Also I will just add here that if your baby uses a pacifier you can get a "baby bag" really cheap. Its a small little pacifier bag that you can stick on your diaper bag.

Some extra things to consider bringing are things like sunscreen, book (one for you and baby), pen and paper (this definitely comes in handy!), a snack for yourself (a moms gotta eat!), toys (age appropriate like a rattle or something small they like).
Some people like to include emergency information in their diaper bag. I mean, don't go carrying your social security card or anything! I would hope that goes without saying, though.

Like I said, it all depends on your situation and needs for what you bring and how much; this is just what worked for me. Good luck and don't forget you will adjust your bag like 50,000 times before you get it how you like it.
Hell, my second baby I don't even use a bag!