Pack for children day-by-day. Use large zipper food storage bags to pack a complete outfit for a single day. At the end of the day replace the dirty clothes back in the bag. Also you can roll these and squish all of the air out so they are air tight and thinner.
Make check lists. Here is a good one that I found here. This will help from over packing as well as keeping a running toll of what you already have or need to get. Or other options:
Delegate a place for everything. When packing up the car make sure you put things where you need them. By starting to think about where everything will go you will also learn where you will have space or no space. Things like a cooler, you need it handy to grab snacks and drinks. Then you have the kids activities, they need to be in their reach.
Keep lists in a binder and keep them close. Make sure to check them as you complete them. If you are staying in hotels print of confirmations and put those in there too. Also have directions and maps in the binder as well. I always like to have a hard copy even though I have a GPS, you never know when it may be wrong or it stops working.