I found these tips from below website and i thought they are really good.
1. Have a Schedule. Set up a general schedule for watching dvd's, playing handheld video games, listening to books on cd, resting, stopping to eat, playing games, etc. Instead of just doing whatever, have specific times when you will do things. It gives the kid something to look toward.
2. Pack a Snack Bag. I have a specific bag with snacks in it. I give snacks according to the schedule, instead of just eating all day long. Each kid has their own water bottle in their cup holder. It has to last for four hours so we aren't stopping to potty every couple of miles. I read on a blog to choose snacks that take a while to eat: cheese packs that you dip sticks into, fruit roll-ups, candy necklaces. I also pack each child a mini-can of pringles and a regular bag of M&M's for special treats.
3. Pack an Activity Bag. In the bag have items for making on-the-road crafts and creations. A sheet of aluminum foil is fun for them to make a creation of their choice. Add some pipe cleaners. Paper sacks and markers are fun for making puppets. Then they have something to play with for a while. Dig through your drawers and pull out any activity pads, coloring books, etc., that have made their way to the bottom of the drawer and been forgotten. I read a cute idea of packing a lunch sack with a new dollar store toy, some stickers, paper, and a snack in the bottom .. a 'grab bag' of sorts. When boredom hits, give them a bag to play with the contents inside. I think this is what will contain the M&M's.
4. A Map. Give each child a map of your route, with a highlighter attached to it with string and tape, so that they can highlight as they go.
5. Money. Give each child $10 in $1 bills. Each time they ask "how much longer?" they lose a dollar. Whatever money they have when you arrive at the destination is theirs to spend on souvenirs.
6. Tried-and-True. Of course bring dvds. We go to the library and check out some videos we haven't seen yet. We have an inverter that allows us to plug things into it for power. So we take our Wii. The kids have got to where they don't play their DS games much at home, so they are always excited to play with them in the car. My kids are really into reading right now, so we checked out some books on cd. Each of my kids gets to fill a small Sterlite tub with a lid with small toys. They love little action figures and things and they will keep them busy for hours, especially Eli. And Kyndal burns us some cds of our favorite music to listen to.
7. The Travel Fairy. This is my favorite thing to do! Each time we stop to eat, I sneak a wrapped gift onto their car seat. When we get back in the car they see that the Travel Fairy has left them something special. It's usually something fun to do on the trip. One of my favorite things is a disposable camera. On this trip I will also give them some sand toys for their last gift, since we will be playing on the beach.
8. Organization. On our Yellowstone trip I bought each of the kids a back-of-the-seat organizer. It holds their handheld electronics, a pad of paper and some markers, their maps, their dollars and anything else they will need within reach. I carry Wal-mart plastic bags for trash, with one attached to the bottom of each organizer. The middle console has a travel container of baby wipes. Brynne has tummy aches that come on pretty fast, so I keep Maalox and a medicine dispenser in my driver's door in case she starts feeling poor.
We really try to drive for about four hours, then stop for a meal and time to stretch. Then go another four hours, then stop again for a meal and time to stretch. And so on. The trip to my parents' in Missouri is 4 hours. So, this gives them a reference.
These tips have worked for us on numerous vacations. If you have any other ideas you have used that I don't have listed, feel free to share!