Over the years I have had quite a few online names and aliases. Some have stayed with me, some haven't. Some should never have existed in the first place because they were just *bad*
Choosing a scene name is one thing if it is only ever going to be used online, but it takes a bit more thinking about if you are going to refer to yourself as it in real life as well or use it as an introduction point at a munch or something like 'I'm ********* on FetLife'
When I first discovered BDSM it was online in adult chatrooms talking to people who I never thought I would meet. Therefore, given I was new and well, to be honest, wanting to get my rocks off a little bit I used very explicit and descriptive names like 'tortureslut4use' and things like that. I actually do cringe to think that was me once. Hindsight is a wonderful thing! I did end up meeting my first dom using a descriptive yahoo handle, and yes with hindsight I'm not sure that was a good thing.
From that experience, I'm not sure descriptive names are necessarily the best. They work for some people but I think there needs to be a bit of subtlety - mine weren't.
So, other ideas for choosing a name :-
Using a name that gives you a special memory
A few years ago when I had the opportunity to be a spanking model I had to pick a first name and surname for my character. I picked the name Ellie Roberts. Ellie being the name of the first girl I kissed and Roberts being after one of my favourite actresses. When I was little when I role-played I often picked the names of my cousins who were older (and who I therefore wanted to be!) or the names of characters in tv shows I liked.
Using the name of a fictional character
I have always liked reading a lot and movies and theatre, there are plenty of names to choose from in the media - whether it be because you just like the name or the aspects of the character reflect something you feel about yourself or your submission. For example I quite like Phedre from the Jacqueline Carey novels I've been reading, or Evey from V for Vendetta.
Using a name from myth and legend
A lot of people on the scene do this. There are so many gods and goddesses and other characters to choose from.
Use other languages
Perhaps translate a word that means something to you or your dom into another language - I know some people have used Ancilla - which is latin for slave. Google translator is great for playing around with ideas.
Use your hobbies and interests
I did some research about other ways of choosing names and another suggestion that came up was to use words from your hobbies and pastimes. For example, if you like visiting a country look up the names of its birds or flowers.
Why I chose 'this girl'
I'm not gorean or anything like that. I wanted to write something only my Dom would recognise, we'd mentioned the 'this girl' protocol in a casual conversation that weekend and I knew he would know it was me and that no one else would. At that point I wanted the anonymity. I still want relative anonymity but many people know who is behind the scene name in real life and across the waves as well and I'm okay with that.
There are quite a lot of people who will only ever introduce themselves as their scene name. Some people have reasons for doing this - privacy, their profession, all manner of things and so this is important to be respected. However, I have never seen any harm in introducing myself at a scene event by my real first name and then 'on fetlife I'm thisgirlsub' etc - if they ask for my online details. I don't know why, I just find it more personal and less likely to appear egotistical to say my real name rather than to come across right away as a normal person that just the side that incorporates my kinky facets. Apart from anything else at events I want to meet people who I might also want to share friendship with outside kink so I don't want to sound as though kink is all there is to me right from the first introduction when that is not the case.
I've known people as well who have been renamed by their dominants once the relationship has got serious and committed, sometimes as part of a collaring ceremony, sometimes not. It's another idea worth considering anyway.
Please feel free to share your thoughts and other suggestions on this topic :)