Hey Flea,
The big thing to remember is to not give an "easy target" signal as well. It can be rather too easy to think you are appearing non-aggressive, when the actual message is "I offer no resistance, hit away". The trick here is to match firm, confident, decisive words and actions with the non-aggressive message.
For example, with your hands, if they are up at about shoulder height with your palms facing out, the message is "I give up". If they are low with the palms facing up, the message is "I don't have anything", if your hands are out to the sides, it is a direct signal of "hit me!", as it shows no confidence at all. Best is to have both hands open, and in front of your body, one hand slightly further in front (creating a barrier). This is a lot easier to show than to write, I'm afraid.
As to the OODA Loop, it stands for Observe Orient Decide Act. In essence, it says that whenever a new stimulus comes into your environment (a punch coming towards your head), you need to first Observe (There's a punch coming at me!), then Orient (It's coming at my head, I need to do something!), Decide (I'll use an evasion and counter punch), then finally Act (Evade, and counter punch). This is a concept developed within the military, popularised by individuals such as Graeme Keurschner (hmm, think I spelt that correctly...) and Jim Wagner.
The concept is not without issues, of course. The main one is that it takes a while to employ, and should any aspect change you start again. So if there is a punch coming towards you, you Observe it, Orient to it, and start to Decide what to do, at which point they see you reacting (or if you take too long deciding, usually due to too many options, you get hit by that first strike), and change what they are doing, you need to Observe what the new stimulus is, Orient to that, Decide again, and so on.
Oh, and a few notes on "mirroring". Don't overdo it, as that can be taken as aggressive, as people can be sensitive to being "mimicked". If they have both hands on their hips, do that, but don't copy every movement. Another use for the term "mirroring", though, is also useful. In this one, you present the opposite to the other person. If they have their right foot forward, you have your left. This does a few things, but tactically, the opponent is presented with a different image than what they are used to. It gives different targets, leaving them unsure of where and how to attack. This is why Southpaws tend to have a great deal of success in things like boxing and kickboxing.
