It may be common sense, but make sure you are nice and clean before you go out. Have a shower, take some baby wipes along to help refresh you as the night goes on, and make sure you are not wearing too much aftershave. There is nothing worse for a couple than inviting a dogger over just to find he is in desperate need of a wash.
Expectations - don't automatically expect to see a couple or some action every time you go out. Dogging requires a lot of patience and you may go out for weeks upon weeks and not see anything. Then again you may be lucky on your first night out. It is a good idea to take some refreshments and reading material along, but don't leave the interior light on all night, or you may end up with a flat battery. Also, don't expect that every couple that comes into the car park will want to have sex with you. Dogging started as a voyeur/exhibitionist type activity and a lot of couples still want this i.e. someone to watch them having sex.
Even though there are no guarantees you will get lucky, it is always a good idea to make sure you are prepared and have some condoms with you. Some couples will take them, but you should always have your own just in case (and take something with you to dispose of the condom afterwards. Baby nappy sacks are good for this).
Reverse into the parking space. It makes it much easier to see who is coming into the car park, and if couples do come along, they would rather see your face than the back of your head.
Signals to show you're interested - each part of the country seems to have different signals for different groups of people, but the one that seems to be universal, is the use of the interior light. It allows other people to see who is in the car and for them to decide if they are interested in taking it further. If you feel a little uneasy about this, try putting on the light and pretend you are looking for something in your glove box. Its lets everyone have a look but is not too obvious if there are also non-doggers around. Get out and stand by your car. It shows the couples that you are interested and is much less intimidating than just walking straight over to them. Stay near to your car, keeping your eye on the couple, and be ready to make your way over slowly if a signal is made (these can vary, but if you are being observant, you will recognise any signals and know that the couple want you to approach).
Signals from couples. As already stated, this can vary, but normally includes flashing or the turning on of the interior light, flashing of head or break lights, but it could be something as simple as opening the window. Don't approach the car until you receive some form of signal to show that it is ok to do so, and then approach slowly without frightening, or sneaking up on the couple.
If you are unsure as to what the couple want or don't want, ask them. Also, even if other people are touching, don't touch without first asking. This may sound common sense, but it is very easy to misread signals, and you don't want to be going home with a black eye.
Afterwards, thank the couple. Even if you did not necessarily get what you want, the couple will remember the polite dogger should they visit the car park again.