It’s not difficult, it does sometimes take time, but it’s not difficult. Just make sure your site has good content and follow the guidelines for adding a URL. Try to get your index page listed at least. I say at least because although ODP claims only to list your index page, there are plenty of sites with 5 – 10 pages listed. So if your site has very distinctive sections, then submit each section – slowly. Once Google updates it’s directory, these listings could do wonders for your sites maximum PR.
As for Yahoo and Looksmart, Pagerank will usually allocate a more than normal amount of PR boost for any sites listed. For tips on Search Engine optimization and getting listed in Yahoo, read Yahoo Submitting Tips at http://www.ibweb.com/blog
If you’re a noncommercial site or have a site that’s almost completely noncommercial you can get into the Looksmart directory through www.Zeal.com. I really love this site, just like Google obtains results from ODP, Looksmart obtains it’s noncommercial listings from the Zeal web directory, without Zeal I would have to fork out hard earned cash and all my site does is provide information.