Tricks

For those who are thinking about doing a similar trip, here you have some useful information:


We booked our flight with US Airways through Viajes El Corte Inglés because we could choose various destinations within the same trip, so it was easier and cheaper to buy all of the tickets (from Madrid to Philadelphia and from San Diego to Madrid).
We hired the travel insurande with Europea de Seguros because the characteristics of their policies met what we were looking for. You can read here the terms and conditions of the insurance policy we chose (in Spanish).
Instead of renting the car from the very first moment, during the first weeks we're travelling by bus with Megabus because prices are amazingly low (as low as $1) if you buy the tickets soon enough.
For the car rental we have chosenTravelJigsaw (through its Spanish subsidiary Ealquilerdecoches), which is kind of a link company between the customer and the actual car rental company. We chose the kind of car we wanted and the website automatically browsed through different American car rental companies and offered us the most suitable company. The price is really good if compared to other companies and even though we had to pay almost twice as much for being under 25, it's worth taking a look at their website (although it's always good to check different rental companies too).
When deciding which places we anted to visit, we have considered personal experience as well as the Lonely Planet guide, which is, for us, essential when travelling. There are, as always, pros and cons about this guide: it's good for backpackers and low-budget travellers (aka homeless travellers), although this one does not include as much information on low-budget travelling as other Lonely Planet guides. However, this guide has the same problem as every other guide of huge areas: there's a lot left unsaid and there's a lot of information that can seem useless for some... As always, it's a matter of comparing with other guides and choosing the best one for each occasion.