I read the post [1] about the uncontinuous blog post id on wordpress. the post gives us a way to disable the auto-save function. Please follow his way.
Anyway, if you want to recover the losing post id, you could use the following SQL to convert the 'revision', auto-saving post type, to draft post.
update wp_posts set post_status='draft', post_parent='0', post_type='post' where post_type = 'revision';
However, the auto-saving function in wordpress since version 2.6 does not affect the performance of sql query severely, but increase the database size.
References
[1] williamlong. 解决WordPress 2.6日志ID不连续的方法. http://www.williamlong.info/archives/1491.html. Sept 10, 2008