1) See what ideas/concepts you have stored in the back of your head, and if anything is applicable in the RP you're building for.
2) See if the NPCs of the world (if there are any) have spaces that you can tie your character to. It's always a good idea to not have your sheet exist in a vacuum. Sheets that feel like real people and have real connections to the GM's world usually do well.
3) It's never a bad idea to make drafts. Don't be afraid of making barebones outlines/trying different combinations with the rulebook. Mess with it.
4) What can you do to stand out from the crowd? What's something unique about your sheet that distinguishes it from Just Another Magus/etc?
5) Don't write something you don't like just to try to get in. It will usually make you miserable. Also, all of the sheets I've had the most fun writing have been accepted. People can tell when you're genuinely invested in something, so try to write something you have fun doing/can imagine yourself playing.