Tips from a good article by Logan Horsford
-- summarized with permission --

1. Brevity is your friend

2. Most character histories follow a tired pattern.  Don't  
    (e.g. parents killed by monster X who stole artifact Y,
     I seek revenge)

3. Writing histories is an ongoing effort.  
     Keep a copy and updated it periodically

4. Do not try to connect your history with current plots,
    keep in generic enough that Plot can do so when apt.

5. Refer to as few NPCs as possible.

6. If you want to include other PCs in your history --
          talk with them.   A lot.    Really a lot.

7. Why are you out adventuring now?  Just being a merc  
     is dull and makes you reactive.

8. Don't come off as evil, annoying, or angsty.
     People assume traits like that come from the player and
     it makes it harder to get started.

9. Do the twist.  (If you parents were killed ...
      they are still around ... as undead villains.)

10. Write someone you can play (and be) for the long haul.    
       Cloaked loners brooding in the corner do not get plot.
       Avoiding plot hooks or sending them to the nobles ---
       you don't get to go on those missions.  

11. Write like a TV Show.  Arcs either finish quickly or run for years.  
        And you don't get to write the other's scripts --
        Plan accordingly
   
12. Be prepared to update things over time.  
       You, the player, probably will change over time.  
       What you find fun may change.  Make sure your character can too.
 
13. Don't clone an existing character from games or literature
        Do you want to play a stale joke for several years?  
        Even if you do plot and the other players will get sick of it.

A good page on general write your backgrounds for LARPs can be found at
http://leavingmundania.com/2011/12/11/how-to-create-a-fun-larp-character/