There are a few basic rules which apply to most sexual situations, be they at a beat or an upmarket swingers' party. Although some people think that they are so sexy that they can  ignore the rules of etiquette, they generally end up not getting any sex because they have ignored the social rules that put people at ease with strangers in these situations. Therefore, a theoretical knowledge and understanding of the etiquette of such situations can help a newcomer fit into the scene more easily. There is some additional etiquette specific to dogging.

Always practice safe sex
Safe sex goes without saying for any sexual situation.

No means no
Everyone can stop at any time. That is not etiquette- it is the law. If someone says 'no' and you don't stop, that constitutes a sexual assault, and you could find yourself sharing a cell with someone bigger than you who may not take 'no' for an answer either.

Wait to be invited to play
Remember that the invitation may not be verbal, so watch for non-verbal communication. Maintain a respectful distance and don't interfere with other people's activities if you haven't been invited; there's nothing worse than someone trying to barge into a scene.  There are numerous ways to fish for an invitation to join in; eye contact, gently touching someone's arm, etc. However, if you receive a negative response, trying harder is unlikely to change it so move on elsewhere- there are plenty more fish in the sea. If someone is trying to play with you and you want to say 'no' without hurting their feelings, you could phrase it as 'I'm flattered that you're interested, but...'.

Be polite and courteous
Be aware of whether or not it is considered rude to stand around blatantly watching a couple play; some people like an audience, but others don't, and some discretion may be required. It is often considered acceptable to watch from a distance. It is also often considered rude to speak or laugh loudly while watching a BDSM scene.  Remember to thank the host as you leave.