This hack lets you set the numbers of posts to show under ‘Manage – Posts’ (tested on Wordpress 2.0+ only). You have to edit the file edit.php in the directory ‘wp-admin’. In this example, I change the number of shown posts from 15 to 30, but of course you can choose any number which suits you. Remember that the larger this lists becomes, the longer it takes to load.

Look for:
$posts_per_page = 15;
and replace it with:
$posts_per_page = 30;

Then look for:
_e('Last 15 Posts');
and replace it with:
_e('Last 30 Posts');