To create a good username, start with the person's name and add a descriptor or use the name of the person's company or business. Another idea is to use a handle created for another website.
Where the username is to be used affects its choice, as different qualities may be desired for a Twitter account versus a dating website. For Twitter, consider the length of the name and how recognizable you want the name to be. If creating a username for a dating website, highlight your positive attributes and incorporate a sense of playfulness.
If you prefer to use nickname, choose something friendly and accessible. Consider is whether you play to use the name at networking events and how it appears on a name tag.
Create a name that is memorable and not too specific. For example, if choosing a name for YouTube, it is helpful in marketing to others if your username stays in viewers' memory. The name will also be around for a long time. Keeping a name a bit broad allows for change and growth in the product behind the user name.
Logistically, it is helpful to use capital letters to separate words in a user name.