1.Talk about your plans. Sometimes as parents we forget that our children cannot read our minds. We spend a lot of time making great vacation plans, but we don't tell them exactly what the plans are, or we gloss over the details. As soon as you start making vacation plans, start talking about them with your children. Even if your children are very young, getting them mentally ready is going to pay off for you. Tell them "we are going on a plane..." or "we are going to be driving far away on a trip..." etc. Get very specific with them. Tell them what you'll be driving, where you'll be staying, where they will sleep, who they will see. You might think they don't understand everything, but that is OK. They will absorb more than you think and it will help both of you be ready.
2. Act it out. As the day draws closer, show your children exactly what is going to happen. Going for a long drive across multiple states? Get some toy cars and blocks and act it out. If you're flying, then driving, or camping, whatever your plans include--show them. It doesn't matter if the toys are exact representation of your trip, use whatever you have available to give them an idea of what is happening.
3. Practice the hard parts. Taking your crew through airport security? Amusement park? Camping? Do a dry run. Set up some plastic bins in your living room and have the kids take off their shoes and jackets and walk through a "metal detector." (set up two chairs for them walk between, etc.) Camp out in the back yard or the living room. Set up your tent and let them get a feel for it. The more familiar things are, the easier it will be when you are on the road.
4. Get protocol in place before you go. Children work best with consistent rules. A protocol for how things happen will bring structure to your trip. Get your child used to a specific way to leave the house, a specific way to get into the car, a specific way to walk when you are out as a family. Get them used to waiting, lining up, etc. An example of this would be having your children line up at the door when you going somewhere, then having them wait their turn to be taken to the car. Having a specific place for them to stand and walk with you when you grocery shop (on your left, etc.) will get them used to staying in a group when you are out. Having these things in place before you go will bring some structure to the chaos of vacation.
5. Attack your anxiety. When you find yourself getting snippy with your family, even before the trip starts, think about what your biggest anxiety about your vacation is and address it. Take responsibility for your anxiety instead of taking it out on those around you. Realize that you can only do so much, and if things don't work out exactly the way you want them to, you still have control over yourself.