There are several things you should consider for a trip, whether you are going on a long or short trip.
Research your destination countries
Before you leave home research where you want to visit thoroughly. Whilst it is nice to choose each morning where you will visit, there is nothing worse than realising that you have already passed something you would loved to have visited. Guide books are an invaluable resource. They give masses of information and tips. There are many websites that offer great information also. Websites generally offer lots of photos about destinations, whereas guidebooks often don't, to save printing costs. See the guide book section.
Prepare your vehicle
An obvious one, but easy to overlook. If you have any problem with your vehicle, get it checked before you leave home. Breaking down is almost always expensive. Language barriers can complicate matters if you need repairs.
Get good maps and even GPS
No matter where you are travelling you will need good maps. Up to date maps are important in places like Spain and Portugal where new roads are constantly being built. Also consider buying a GPS unit. They make navigating very easy.
Take lots of music
Make sure you take lots of music with you. Take lots more than you think. If you are spending lots of time in your vehicle, whether travelling or stationary, you will get through music quickly. If you have a MP3 cd player in your vehicle make yourself lots of CDs using this format, as it reduces the number of CDs to carry.
Take books in your preferred language
If you are a reader and are going to county where your preferred language (such as English) is not the first language (in a country such as Portugal) take a few spare books. Charity shops are a good place to find lots of cheap books. Getting hold of a foreign language book is not always easy. The best place to get hold of a foreign language book on your tip is in a large bookstore, especially in cities. Some campsites have a bookshelf where you can swap the books you have read with books other people have left behind. Also ask other campers you meet, to see if they have books they would like to swap.