Here are some things to consider when you're putting together your game call.

1. Right off the bat, hit on your style of play.  (Dedicated and committed GM looking for like minded players.  And anything that you feel should be shared about your GMing style).  It's a leap of faith for players to create a character, and the more they sense that you're committed the more they'll be willing to jump through any submission hurdles that you set up.

2.  Information about the game.  At least one - two meaty paragraphs about what kind of vision you have and what you'd like to explore in the game. 
a.  Genre? 
b.  Books, movies or television programs that may have the right look or feel of your vision. 
c.  Mention any character criteria or exemptions, anything you won't be allowing.  Help the potential talent focus their energy on ideas that will get you excited.
d.  Also game system information if this pertains.  Free Form is fine, but if you do have a system to use as a backbone, let people know what it is, and how important it is to you if they know it.  (For example, I use Hero, but I could care less if my players know it, as long as we can communicate through the creation process.)

3.  Posting Requirements.  (I think this is critical.  Let prospective players know up front what your expectations are in this area.  Minimum post of one, two, three, whatever times per week.

4.  Submission Information.  What is your ideal number of players?  Tell the perspective players how you want your submissions (text, .doc, etc.), and what you want (Character bio, writing sample or link, etc. etc.)

5.  Contact information.  Most importantly where to send it.  Your email address, or whatever your preference is.  Also, let them know where to send questions, via site PM or email, or whatever is good for you.

6.  Deadline.  This is the easiest to forget after writing all that other stuff.  But remember to tell people when the deadline is.  Us gamers tend to be procrastinators.