The most tried and true way is to request links. If your website's link would likewise point back to such websites, it may even result your website to land to quite a low page ranking. When exchanging links with a different site always make sure that the website has a greater PageRank than yours. When you get your site URL on somebody else's website this is called a backlink to your website. Also your site would acquire a good rank.
Considering all the advantages that SEO has to provide a business, you need to present these advantages and justify the money the customer is paying.
The most tried and true way is to request links.