I've had a lot of success by adding valuable tools that I know my visitors can use. I am in the telephone answering service business. All of my clients use call forwarding, so we provide instructions on call forarding for every telephone company we can find.
I also recently added a tool for sending SMS text messages and vairous pages describing how to send SMS via email and visa-versa.
I was told by someone who knows more than I to always try to deliver relevant value. In the beginning we tried the usual tricks like weather forecasts and recent news. The items I mentioned above increased our organic traffic and more important, reduced bounce backs.
To retain a customer, keep him happy. Because they are satisfied with your service, they'll be happy to give you referrals. But, of course I know its easier said than done. It depends on industry, you can offer incentives and let them know they are appreciated!
I think you should rephrase that question. The word trick should not be even considered into customer retention. If you're tricking anyone, you're not running a business.
Don't scam your clients/customers/visitors. If you provide them with the best possible service you can provide them and it's a viable service that does address their needs, then you'll have no problem for customer retention.
If you're not selling anything of particular importance that there isn't demand for, don't push it on people. You're better off closing up shop and saving face. Remember, your reputation is the only thing you possess that is eternal.
Remember that you should build a remarkable and efficient brand name and website. This will help you to gain trusts from people and turn them into good clients. Also, you should keep them (people and clients) updated and provides their queries. A company that has good relationship among of their customers or to people in the target place wins the market.