Pick only the most appropriate category. If you believe your site should be listed in more than one category, only submit your website once and indicate on the form while submitting what the other categories are. Try to limit yourself to only one additional category.
Yahoo wants to offer unique sites. If your site has nothing new or different from the other sites already in Yahoo, chances are you won't get listed.

People list Yahoo. Yahoo is not like other search engines, an actual person will visit your site and determine if your pages are worth adding to Yahoo. Make sure that what you have to offer will appeal to Yahoo's users.
Get popular. If your a popular site (ie: lots of other sites link to you), then your site will become more noticed and all the major search engines (including Yahoo will begin indexing you).
Avoid sending multiple submissions, submit once and wait at least 2 months before trying again. Swarming Yahoo with submissions will only get you blacklisted.

Get regional. Try submitting your website to a section of Yahoo under your local state/province or city. Chances are, the editors in these smaller sections of Yahoo are looking for community involvement and will list you.
On a final note, getting listed in Yahoo! can sometimes take up to 8 weeks. Yahoo! is by far one of the top search engines and as such takes a very long time to get listed so above all, have patience.