Which category?
The Open Directory Projects data is organized into 16 top level categories each with many many subcategories, however only 15 are visible from dmoz.org as the adult category can only be accessed direct from its URL, this is of course for the protection of minors.

It's imperative that you submit to the most appropriate category for two main reasons. These being the fact that if you submit to an inappropriate category the editor will most likely reject your site and move on and if you don't submit to the most appropriate category you may have lost some valuable keywords from your category name. This last point is important as the Google directory and indeed dmoz.org themselves not only search titles and descriptions for keywords but they also search within category names and paths, so having your keywords in your category name will help you be found.

Deciding on which category to submit to isn't that hard, what I suggest would be to visit dmoz.org and enter in your primary keywords and or keyphrases and see what results come up. Categories which contain returned websites will be listed at the top of the page, one of these categories will be the one you should submit to. Start by eliminating those categories which you know are not right for your site, for example foreign language categories and other categories which although they contain websites returned from your search do not fit your sites theme and hence keywords very appropriately.

Imagine why don't we that there are three categories which seem really right for your website and your submission, how do you decide which one to submit to? Well when I'm submitting any site and come across the same problem there's a couple of things I do to help me out.

I always check if there's an FAQ and description page associated with a particular category, within these pages are specific category guidelines concerning what sites the editor deems highly fit for his or her category among other things. If you submit to the right category your much more likely to get accepted and this information will help you decide on the 'right' category. Not all categories have FAQ and description pages however.

I always check if there is in fact an editor for any categories I'm considering submitting to as editors of parent categories are often too busy to deal with subcategories of theirs without their own editors. I also scroll to the bottom of the category page and check out the 'Last update' date, if the date is fairly recent obviously there's an active editor for this category as opposed to an editor that rarely updates and looks after his or her category. I do both of the above as I'm a businessman and time is money, if these checks return unsatisfactory results chances are I'm going to be waiting months at least for a listing in that particular category so I might as well submit to another one.

Often I use the handy 'little green ball' feature on dmoz.org to help me decide on my category. Let me explain, the 'little green ball' feature as I like to term it is simply a graphic (the graphic is of a little green ball) hyperlink from a dmoz.org category to the equivalent category in the Google directory. ODP does this as it provides users with the chance to see a categories sites listed in order of decreasing Pagerank instead of dmoz.org's standard alphabetical ordering. I use this to see what the competition in a category is like, Pagerank wise that is. Googles directory displays listings in order of decreasing Pagerank by default, so sites with high Pagerank will be displayed at the top and hence get considerable more traffic than sites listed near the bottom. Taking this into consideration I would always try to submit to an ODP category which hasn't got too many high Pagerankers as I would most likely be buried in the listings and hence receive very few visitors from Googles version of the directory. Remember the Google directory does get used an awful lot too, even though Google search is very accurate and powerful. The green ball link is located at the bottom right of each category page.

Now for a note regarding regional operations and category Pagerank. If your business is region specific you should always submit to a category dedicated to that region as this will help you get very targeted visitors from dmoz.org, Google and the other sites which use ODP data. As for category Pagerank that refers to the actual Pagerank of the ODP category in dmoz.org which your considering submitting to, remember a link from within a category with a Pagerank of seven, is more valuable than a link from within a category with a Pagerank of five, in Googles eyes anyhow.

When you've decided on your category the submittal form is found through the 'add URL' link located in the top right corner of each applicable category page. Now I will discuss each aspect of this form.

URL
In the URL field type the full address of your website. This may seem obvious but always spell check your address, this means not just the domain name part but also the http:// and the TLD. Numerous sites have been rejected by the ODP for not bothering to fix mistakes like using http:/// instead of http:// and similar mistakes. This is downright unprofessional and in my opinion editors are right to reject sites based on URL field mistakes.

ODP title tips
During your submittal the ODP will ask you to enter a title and description for your websites listing, these are the most important elements of your submittal, do them right and you could be in for some good traffic courtesy of the Pagerank boost Google usually assigns sites listed within ODP, however do it wrong and you could be rejected immediately.

Regarding the title you submit ODP stats you should "Always opt for the official name of the site". This backups another ODP quote from its official editor guidelines page located at http://dmoz.org/guidelines/describing.html "The title should identify the site, not describe it. It should be both informative and concise."

Using your official business name or website name will indeed identify the site and hence it'll be informative and concise. Always do use your official business or website name (often the two are the same) instead of including some keyword filled promotional hype with a junk of alphabetically high characters located at the start in a bid to get near the top of the listings as if you do this the editor reviewing your submission will reject your site.

Don't forget that the primary benefit of a listing with the ODP is not traffic from dmoz.org itself but from third party sites which use ODP data. One of the most prominent sites using this data is the Google Directory and by default Google directory displays listings arranged by Pagerank so there really isn't much point to including characters like AAA at the beginning of your title anyway.

Having included your official business or website name in your title, you may be tempted to add something like 'Welcome to', 'Homepage of', 'Website of' and other similar phrases, however don't do this as editors are advised to remove phrases like these so it's just a waste of time.

Nearly all titles I have looked at in the ODP have all the first letters of all noun words capitalized, this is obviously the way Open Directory Project editors like it so if your title is a couple of words long I would suggest you do this. Be careful here not to capitalize everything, but just the beginning of noun words, don't capitalize words like 'and', 'at', 'of', 'on' and the like. This tactic will make life easier for the reviewing editor which helps greatly towards a final acceptance of your site. I'm about to move onto discussing the description but first here's a page I recommend http://dmoz.org/erz/sites/title.html it gives examples of good and bad titles and explains why they are so.