Your client is after 5000 unique visitors per day.
In order to achieve that target have they fully researched their market and are sure that there are at least 150,000 queries per month around their topic (that is the very minimum number of queries to consider but the actual number of queries will need to be much higher as you are unlikely to enter directly to #1 rank or become highly visible too quickly).
In addition you would need to ensure that the site is SEF (Search engine friendly) and that the site is easily navigated by people - so they stand a chance of finding what they are after once they arrive on the site (all internal links are working etc).
After that you will need to promote the site through various methods:
1. Directories - there is no sense getting listed in 1000s of directories if they are not passing on PR or real visitors. Free listings can help a very small amount, but not much in the overall scheme of things so marketing will need to account for the costs of a few well selected authority directories.
2. Article Writing - Submitting articles with relevant text links can help identify the site to search engines and the expected content that would be found. It can help increase visibility of the site but is not an overnight solution to 5000 unique visitors per day, it is only a part of the scheme of things.
3. PPC advertising - This could get you the 5000 unique visitors per day quite easily without the need for other methods. However, it will be expensive and depending on which area the site is in, it could be non-profitable.
4. Direct Advertising - Approach related sites or authority sites that have the same target audience but are not direct competition. This will ensure more controlled costs although it can still be a significant financial investment.
5. Viral Marketing - If the website is useful with either a product or service that is worth talking about, get it into the blog world and get people talking about it. This could involve approaching high profile 'professional' bloggers who get paid to review or mention or link to the website/product/service. - The viral aspect kicks in when other bloggers read the information and then start blogging freely about it which can bring some surges in visitors but doesn't guarantee 5000 per day.
That is just 5 methods that could be considered in addition to on-site SEO through being SEF and well structure as wel as additional off-page SEO through selected one-way links other than just directories and article sites.
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