Keeping personal privacy in mind, having a clean user name and being ready with several alternatives in case the first choice isn't available are all tips for choosing a good user name. User names are ubiquitous across the Web, and if desired, users can link dozens of services by using the same name for each. Users should think this plan through to avoid future regret.
Small variations on the same general theme can link user names across different services, so using a completely different user name for every service is a great way to keep people from following a digital trail. If a user name has any dirty words or foul language in it, this may reflect poorly on the person behind the user name, and it may cause some services to forcibly change the user name if it's offensive to other users.
In some cases, a user name that is available on one or more services may not be available on others. Being prepared with multiple user names can be helpful and save time in case the desired one isn't available on a certain service. Adding numbers, underscores or a few extra letters to the beginning or end of an already taken user name can generally result in one that is available for registration.