I certainly don't know everything there is to know about SEO, but I was recently made aware of the duplicate content penalty. In other words, search engines will penalize your site if your domain aliases are simply displaying a copy of your site rather than redirecting to your primary site. To work around this, I have generated this bit of .htaccess code to redirect any domain that is not your primary domain to your primary domain.

# BEGIN SEO Redirect
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{HTTP_HOST} !^(www\.)?nextech.net$ [nc]
RewriteRule ^(.*)$ http://www.nextech.net/$1 [r=301,nc]
</IfModule>
# END SEO Redirect

Change the above snippet to match your site's domain and insert it into the .htaccess file on your site's root. Now your site should have a search engine friendly "301" redirect to your primary domain name.