The best way to boost the performance of your website in search engine results is to get more backlinks from websites. While a backlink from a directory is not a very valuable link, submitting to directories is still one of the easiest ways to build links. Directory submissions will never get you to the number spot on Google for a very popular keyword like "free software", but for a low-competition keyword, directory submissions can still work well.
The usual advice is to do manual directory submissions. While this is plausible for a small number of high value web directories, the amount of time involved in making more than 10 or 20 directory submissions by hand means that you'll soon need the help of directory submission software. If you choose to use a directory submitter, make sure that the submitter lets you manually choose directory categories, or at least is itself very accurate in selecting the appropriate category in a directory to which to submit your site.
If you don't submit your site to the most appropriate category you are devaluing the time you have put into submitting since the directory owner will be more likely to reject the submission as spam and as your website details are out of context the worth of your backlink will diminish.
Submitting to directories is therefore a compromise between getting value and spending time. Quite often, you'll see advice about paying for someone else to do the manual submissions to directories. The problem with this route to submitting is that you can't be sure how reliable the manual submitter will be. In fact, you have no guarantee the person isn't using a directory submission software package to perform the submissions. Most submission services are so cheap, it's difficult to see how the service could operate without the use of a submission software package. The best advice is to perform manual submissions to the major directories like DMOZ then use submission software to submit to lower-quality free directories, saving yourself loads of time. And always remember that directory submissions should only be one part of a search engine optimization campaign.
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SliQTools develop high-speed, easy to use directory submission software . The most popular directory submitter is SliQ Submitter Plus.