First on the check list is diapering and clothing. Following are item you should not forget: travel size container of 6-8 diapers this depends upon how long your outing will be(plan on one diaper for every two hours), diaper wipes, changing pad to keep things clean, Ziplock bags for diaper disposal and soiled clothes, 2 or 3 changes of clothing - depending upon how messy your baby is, hats which are appropriate for warmth or shade, , diaper rash ointment, baby powder.
The next category would be anything having to do with feeding: if formula feeding you will need bottles, burp cloths, jars of baby food and spoons, 2-3 bibs, cans of formula or some containers of pre-measured formula, bottled water for you ( also for baby if using powdered or concentrated formula ), portable snacks for you and possibly your other children.
Continuing you need to have toys and soothing items with you to keep your little one happy, occupied and quiet. Here are some things you should not forget: pacifiers, teething rings, 2 blankets, comfort toys, small durable books (board books are great)
Preparing for your baby's safety and emergencies should also be a priority on your list. So For long trips here are some things that you should not forget to put in you diaper bag: band-aids, antibiotic ointment, tissues, fever reducer/pain reliever, hand sanitizer, cell phone, and sunblock.
Finally here are some diaper packing tips. Pack a lot of diapers. For every two hours that you are out you should plan on packing one diaper. If your baby has diareha then you should think about packing some extras. Consider the weather conditions and then choose accordingly. Some people forget their keys often if this is you then you should put an extra set in the bag. This is a great idea if your car auto-locks when the driver's door shuts. This extra set of keys will come in handy if you accidentally lock the baby in the car.
If your child has allergies, you should always carry a liquid antihistamine in the diaper bag. For severe allergies, you may want to get an Epi-pen by prescription so that it can be kept in the bag at all times.
For those special moments you may want to keep a disposable camera with you in the bag. Also you should think about putting it in a bag to protect it from spills inside the bag. If it is not a disposable camera then you need to be extra careful.
If your formula feed, you are now able to use powdered formula. This is very convenient. All you have to do is measure the powdered amount of formula in an empty, clean bottle, and then pack a few bottled waters and mix it with the formula when the baby is ready to eat.
This is an excellent checklist to go by. Best thing to do is always have a diaper bag ready for any time that you need to rush out of the house for something. It can be difficult trying to pack a diaper bag at the last minute. This will ensure that your outing goes smoothly!