Gwyn Gilliss has an informative piece on the topic in Backstage this week (you can see the full article here) that gives useful guidelines that sound right on the money. I will say that having been a professional writer I still managed to miss most of this, so my next task this morning is rewriting my bio.
Here's Gwyn's step-by-step advice:
Paragraph 1: Recent roles/Strongest credits. (Theater if you’re in New York and film/TV if you’re in L.A.) Try to use recognizable plays and roles, not just “showcases.” If you’re just starting out, you can include “representative” roles. Those parts from Shakespeare or Chekov done at school outweigh showcases of unknown writers Off-Off-Broadway.
Paragraph 2: Training. Don’t be afraid to name drop master teachers or prestigious drama schools, as well as directors you’ve studied with. If you’ve worked with “greats,” they will assume you will be great!
Paragraph 3: Recent work. (Switch what you included in paragraph one.) Include Indie films and appearances on primetime or daytime TV or include all major stage credits from Off-Off-Broadway to Broadway. Your credits tell them how to cast you and what roles you are consistently hired to play. Don’t include extra work—it's not considered a professional credit if you’re standing in the background.
Paragraph 4: Personal Life. Here, write about your interests, skills, travel, languages, or musical instruments—anything that makes you memorable. Elaborate don’t just list.
She also weighs in on style, advising actors to keep it short, avoid lists, give the "what" not the "why," and write in the third person and in inverted pyramid style that puts the most important information first.
I would only add that somewhere in there - probably up near the top - you find a way to work in the three on-screen qualities that make you compelling as an actor. For example, I'm often cast in power roles - judge, corporate executive, member of Congress - so my three qualities are "forceful, intelligent, pragmatic." It can help in casting.
I would also advise that you downplay training as you build experience. Once you've gotten recognizably good roles, training becomes less and less important.