What happened in here? Clean this mess up!
If the outside looks good, that's one thing. But people actually ride on the inside, so you need to keep that looking good too (throw those old McDonald's bags out already)!
The interior should at least be quick dusted and vacuumed every 6 weeks. Windows can be stretched to once every 2 months, unless you are in the habit of touching them all the time! To keep the interior stain free, clean it every 4 weeks or less.
Pull out your floor mats. Rinse and wash (car wash soap again) rubber ones. If yours are carpeted, vacuum them. If they are stained, you can use Resolve carpet cleaner on it. It works really well. If that stain won't come out anymore, maybe you should consider new ones. Has it been over 2 years? You can also wash the carpeted floor mats and shampoo them, but you need to make sure they are completely dry before you put them back in the car (hang them up for 10 hours in direct, warm sun) or else you might get a damp, moldy smell a couple weeks later!
Dust the interior with a damp rag. This picks up all the dust without just throwing it around. Vacuum the seats and carpet. Any stains? You can use the Resolve carpet cleaner mentioned above. It works wonders sometimes! Be sure to get the areas under your seat, and don't forget to vacuum the trunk!
Once the floor and seats are clean, use a protectant (Armor All et al) on any vinyl, rubber, or plastic trim including the dashboard, door panels, and seat trim. Be sure to spray the protectant on the rag, not on the parts it's being applied to! Keep it neat! Also, try to keep the protectant off the radio and other controls. You can do your steering wheel, but don't do the outside where you have to hold it - your hands may slip when you don't want them to! Besides, you would just rub it all off with your hands when you drove anyway! Don't forget to do the shift boot (5 speeds) and the center console, as well as the kick panels and the plastic carpet runners!
Clean the glass inside with glass cleaner and paper towels. Also clean any chrome, simulated wood, and gauge cluster covers with the glass cleaner.
Leather interior owners may want to apply a leather protectant of some kind when the interior is all cleaned.
Are you done yet? If your exterior is clean, this is the end!