If you're looking to get a directory to the top of Google you need to focus on one thing:
Quality
I see too many directories today that are completely unmoderated and they accept every site that gets submitted. Begin by developing a site of Quality Guidelines and Standards for your directory and follow them. If someone submits a site that doesn't seem to fit in: remove it. Don't allow people to place links in the top directories; require them to place sites in the specific category or categories.
It's often a good business technique to sell featured links because this helps those website owners get more exposure and it helps you pay for your hosting and directory software. It can also be considered compensation for your time spent developing and maintaining the directory.
Never deny sites that have a low
PR because everyone starts somewhere! Finally, work on good promotional techniques and backlinks and that will help dramatically.