1) You can check your theme files using a theme check plugin – http://wordpress.org/extend/plugins/theme-check/
2) Make sure you are using the right way to enqueue external scripts.
3) Make sure you include the basic wp hook within your theme.
4) Do not use php short tags.
5) Before any release you should turn on WP_DEBUG in wp-config.php to fix PHP errors and to see deprecated function calls .
6) Load external scripts ONLY when needed for a page template.
If you have any other tips would like to share with us, feel free to drop me an email and I will add it here. There are so many can be included here but above are those I think is important.


Thanks for starting this thread, Kailoon!