Keep your voice(s) down
You know who screams out in the middle of the night? People having orgasms, and people being murdered. If a passerby hears the former and thinks it’s the latter, they just might call the cops. Don’t be a screamer.
Keep your interior in mind
Subaru Outbacks are ideal: fairly spacious in the rear, and capable of handling dirt roads... which is probably why my super-slutty Portlanders drive ‘em. Not just for camping and lesbian farmers!
Auto expert and writer Jason Torchinsky wrote a great piece that breaks down your optimum positions, depending on the type of your vehicle.
Keep essentials on-hand: tissues/baby wipes, water, lube, and chewing gum
Since oral sex is the most performable maneuver while in a vehicle, it makes sense that you’d want to have a handful of tissues in the side pocket. Nobody is obligated to swallow, and I know I don’t want to clean the aftermath out from under my bucket seat.
If any bodily fluids do get on the interior, leather is way easier to wipe -- and is partly why I chose that interior for my most recent car purchase.
As always, don’t litter
Think about the children! And the baby birds that might choke to death on your used condom. Don't toss your trash out the window upon departure. If you're not conscientious enough to care about the impact on the ecosystem, at least be mindful that when you leave evidence, you’ve just given away your spot.