How can I submit my directory listings to loads of search engines at once?

There are a lot of misconceptions and half truths about search engines and how to optimize your site for them which have already been included in the answers I've seen so far.

First off YES, you can submit your site to multiple search engines at once. People usually use search engine submitters/blasters for this. There are a few services that will charge you a fee for this----don't be fooled by them use the FREE services.......................
Why?

The majority of these submitters may not submit your sites to the search engines that matter in terms of usability by internet surfers (in fact some require manual submissions to work correctly) and which will provide the bulk of the traffic to your site. It was found by the Nielsen Net ratings in 2005 that some 90% of search engine traffic originated from

Google
Yahoo!
Msn
AltaVista
Ask.com
All The Web
WISENut
HotBot
Gigablast
(It hasn't changed much since 2005)
So if the blasters aren't submitting to these your missing out!!
In fact the first three took up some 80% of the use by surfers!

There are also some other high ranked ones like

Ask Jeeves
Webcrawler
Iwon
AOL
EXcite
Netscape

But the truth is that a lot of the lower ranked search engines (SEs) (even some of the top ranked ones) use search results from the wider used "robots" of Yahoo!, Google and Inktomi/HotBot. So why ask a submission service to submit to
multiple SEs (I've seen some for 100/200 submissions)
when results for the lower ranked ones come from the top ranked ones?

Try using Submit Express
http://submitexpress.com
They also provide other useful search engine optimization (seo) tools lilke meta tag generators and keyword suggestion tools.

Secondly, many people feel that once you have your site up with links to other sites and a sitemap you will be indexed by the search engines. I don't agree with this! What are the quality of these links that you have to your site? Search engine robots take a lot into account when choosing which sites rank well with them. One of these things is the page rank of the site linking to yours and correspondingly the page rank of the other sites linking to your link partner.

So not just because you have a link means this will mean good for bots searching for your site. In fact, some of these other sites will have sitemaps and robot.txt files which tell the search engines where to index on their websites. How do you know your link is included within these files for the search engines to crawl?

It is best to submit your sites regularly every 6 weeks depending on how many changes are made to it and do it yourself manually plus with the aid of the free service I mentioned before. Don't submit more often than that as your site will be penalized for "spamming " the search engines. Also submit not only to search engines but search directories as well. The most popular of these is DMoz (Direcory Mozilla) also known as the Open Directory.
http://dmoz.org