First, how unique is the name you are contemplating? Does it matter to you if you could be attending an event with others of that name? In period there would have been quite a few "John Smith"s running around, but how practical is that in a society where we already have two names to have the same name as someone else? Or be "John Smith called Spinny" and have people not recognize when a herald is trying to call you into court with your formal name? My advice is always to go for a unique name. At an event in my local shire there were 5 Mistress Elinor/Eleanors, and that was before House Strangewayes got our own! Don't add to the confusion!
Pro tip: Try starting at the bottom of name chart and work your way up!
At Herald's Point at Pennsic they keep binders of the registered names. The alphabet starts with HUGE binders and drops off as the letters go on. There are so many wonderful, unique, and perfectly period names out there! As to popular names, anecdotally, - any variation of Cat or Kitty whether Catherine or Caterina has dozens across any area. And don't be another fighter named Angus. Just don't.
When contemplating a name, first check this chart of the most common names in the Society:
Remember that your first and last name should match both in culture and time period. The Society allows some leeway on time period (I believe it is about 100 years) and culture, but names generally "used only one language, or sometimes two. Most people didn't travel far from home, and knew just the one language they grew up with. The few who did travel did so mostly to make war or money (or both); rarely did they travel to settle in some distant, inhospitable land. As a result, names with two languages appeared only near kingdom or national borders, or as a result of major invasions or regular trade. Thus, it's reasonable to combine English and Danish, or French and German, because those people ran into each other regularly. On the other hand, mixing English and Arabic, or Hungarian and Irish, isn't reasonable, because those people interacted only rarely, if at all." from:http://heraldry.sca.org/armory/whatis/name.html
Then comes the heralds. The pronunciation of your name (or the butchering thereof) is more than a little likely if you are unknown to a court herald, or if they haven't seen your name written. So, unless you are my dear friend Þjóðrekr ógæfa and think that this is a fun trick to pull on the herald, try sounding out the names as you go down (or up! Pro-Tip!) the lists. This may make you look like a crazy person to co-workers, but will serve you well in the end.
Now, in my household our Founder Eleanor Strangwayes now Iulia Agricola will tell you that a joke name will eventually get old. As Fortune, I won't tell you that. But that is something to bear in mind. This will likely be your name in Society for years to come and looking forward how will it "age"?
Also, as many of us can tell you, there are people who will no longer know your "real" name and you may get stopped in the grocery store or have your SCA name called out across a crowded room at a convention or on the street, and how will it go in those spaces? Your name, just like your mundane name, will be what people judge you on without knowing you as a person, so choose thoughtfully.
Another issue I've seen happens when someone chooses a descriptive name and then they change focus. This is very evident in heraldry, and the Herald's Point staff will caution you that just because you like archery - doesn't mean you have to have a bow or arrow on your heraldry.
Your heraldry, like your name, will come to mean *YOU* it doesn't need to define you.
Later down the road Malcolm Bowman who once was a fletching young archer is now Master Malcolm Bowman a well-known herald of the East Kingdom.