1. If you are crossing a stateline or border, stop at the welcome center. You may feel like an awkward tourist, but if nothing else, you will more likely than not be able to snag a free map and use a clean restroom.
2. Bring paper maps. (See tip #1 for obtaining them free of charge) Aside from the joy of getting to spread it out on the dash, you will be able to see what lies ahead road/park-wise as well as discover the history via town names. These maps come in quite handy if your phone loses power or towers drop out of service or, heaven forbid, the 3G stops working. Or download them before you leave off the state's tourism website.
3. Bring water/water bottles. If you don't feel right purchasing water bottles in bulk, bring a refillable water container so that you can keep hydrated. Sitting in a toasty car all day can leave you parched, so drink up.
4. Get out and ask questions. Whenever you stop for gas or even a break, walk around and talk with the locals. If you're buying gas, ask questions about the area or the furthest they've traveled. You may just some interesting tidbits that tripadvisor/the guide book left out.
5. Unplug, at least for a few hours during the trip. Nothing kills a trip like a fellow passenger being plugged into their phone, earbuds, ipad, whatever. Twitter will be ok if you don't tweet about the cool sunset, tourist town, etc. You are out there to experience it, so get to it! Maybe you'll be a daredevil and forgo the radio too (you'll be surprised how much more you can take in and bounce around with your road trip buddies).