[@Aura] One of my suggestions, and how I did many of mine, was to do a timeline of events for the bio. I would basically start from my PC's birth, determine what events were important to her (like say key milestones like first date, prom, sports she or he liked, etc then progressed into adulthood and finally into the present). It helps with fleshing and writing about the PC's bio while preventing it from becoming lengthy or complete fluff, if that's what concerns you.

Btw, I'm not aiming to step on the GMs toes with this suggestion, merely thought I would share my tactics to help.