The most important thing is getting more eyes to your listing. One of the ways you can do this is by promoting your sale to your targeted buyers (as you did when emailing potential buyers), letting your site's visitors know that the site is up for sale, sharing the link on forums/blogs/social media, etc.
I mentioned featured listings and other upgrades before, as upgrades boost visibility and sale price (see http://blog.flippa.com/guest-post-what-factors-influence-you... for some data on that). The duration of these upgrades depends on how many other people apply the upgrade; generally, Featured Listings are on the front page for about 12 hours. Generally.