Also...
in reply to a message by Aine
I think as long as you have another nickname lined up to give people an alternative, you have a better chance. For example, I like Alexandra but hate Lexi...if she is established early on as an Alexandra occasionally nicknamed Allie or Alex or something, I'd feel more confident in her never becoming a Lexi. If you go into it being 'She is Alexandra and that is all she will be', there is a much greater chance that it will get shortened to something you have no control over in high school (although it isn't inevitable).