Getting potential clients / customers to your site is only half the battle. Once your have them there you have to keep them there.
Give them what they want. Cater to there needs. Ask them what else they would like. Be sincere in wanting to be of assistance. Write compelling persuasive content that will keep them focused on their initial purpose for visiting your site.
To build your opt-in list you must do the same. Offer useful information and content that is unique. If you expect someone to give you an e-mail address you have to offer them something that they will find useful. The ultimate goal is not to sell your product or service, it is to build a customer /client relationship and trust.
Remember, if you are an e-mail marketer you must adhere to the Anti-Spam Laws.