Great questions, Mistress Ahn. I think this varies from person to person, but I'll put in my 2p for what it's worth.
In general, I start by looking in my area, but since my criteria is rather selective, I rarely find anyone of interest. I then proceed to look within my continent. Then anywhere in the world. My primary resources for searching are google, alt, various professional or lifestyle community websites, etc. I came upon FortressNYC after finding Mistress Heung's website, and would have been more than happy to pick up my entire life and move to NYC if it meant I could have sessions with her, and to be honest, to hear of her retirement from the Fortress was a little bit heart breaking even though I've obviously never met her. Finding a mistress is a huge deal and can involve relocating, finding a new job in a new country, even learning a new language (unpleasant experiences, to say the least). Of course, it varies from sub to sub, but I consider myself a sub first, and a human second, if that makes sense, so it follows that finding a mistress is one of the most important things in life.
There are some spectrums and some compatibility factors to consider, however most of this takes place on a purely intuitive level. For each sub, the importance is placed on different things. Again, going by my own experience, I put a great deal of importance on ethnicity/culture background (which has an all-important impact on appearance), demeanour and shared interests. I can see the makings of a formula in my selection process, however it's intuitive, not intellectualised. It could probably be extrapolated into a mathematical formula, but like all discreet functions of this nature, such a formula would make calculus look like child's play.
Appearances are the single most important factor for the initial attraction. Getting past that and onto compatibility depends on other things, but everything is still filtered through appearances. That's why you girls can get off traffic citations so easily, or get discounts on anything. If a guy looks into a girl's face and he sees heaven and all that is good in life crystallised perfectly into an angelic being in front of him, then there's not much left for the mind to decide on. Everything is taken in a positive light. So is it enough to go on? Yes. That's the curse of being male.
This is really splitting hairs. How to decide between multiple girls that all 'make the cut' as it were... well, suffice to say, usually only one girl is really the apple of a guy's eye, at any given time. Even if there's a double-team going, one girl will always seem more attractive than the other. It's not just one thing, it's a systemic filtration process that begins with the first moment you see the girl. The condition you are in at the time, her appearance, what you're thinking, what she's doing, everything. Chaos theory is a perfect fit with human attraction.
That said, well taken photographs do work wonders. As do cosmetics, angles, lighting, composition, and the perfect alluring expression. But a bad photograph of one's ideal girl will always prevail over a great photograph of the second-best. Maybe it's different for other guys though.
That plays into the picture after the girl has already pretty much been decided on. Seeing great reviews just makes you more confident in your choice. Seeing bad reviews will just make you think that everyone else has bad taste, and you'll enjoy setting that all right with your raving review later. This might have something to do with why bad reviews are so few and far between.
Each sub is different. To me it is relatively important that she can get into my head and take over. Intelligence is important, but not nearly as important as the application of it. If a girl can predict everything I'll think, do or feel, and somehow use that against me, and then find amusement and pleasure in that, then I'm hers. Of course, there's a huge turn-off for me when a girl thinks she has to act like a guy in order to be dominant. Loud shouting, brute force, crude language, and that sort of thing create a bit of gender role confusion, and then it's not like you're being dominated any more, it becomes more of a thinly veiled act. All qualities that preserve femininity, while furthering the extent and ability of domination are valuable.
A lot of research goes into finding the right girl in the first place. Several years in fact (a lifetime for some). And then once you've found her, it doesn't take much research, you just know. I'd say, 2 or 3 hours altogether, after you've found her.
My ideal mistress is Korean (Korean born and raised), symmetrical, extremely confident, interested enough in me to learn enough about me to effectively dominate me, takes pleasure in sensuous intimidation and exceptionally skilled at humiliating me in ways that will enter my head over and over again for the rest of my life, able and willing to take it to the extreme, even possibly to put my life at risk for her amusement (if she so desired), considerable quantities of sass, some interest in edge play, unpredictability, an ever-increasing appetite for power and control.
Hope this answers some of your questions.
