Detail Your Interior
Take the time to detail your interior. Nothing looks worse than a car that's clean on the outside, but looks like a McHurricane hit it on the inside. Toss out those fast food wrappers, vacuum the carpets, use some leather or cloth cleaner on the seats.
Forget to dust the dashboard and instrument clusters. That dust may not be very obvious when your whole car is dirty, but it will stick out like a sore thumb if you clean everything else and forget about that.