I don't like the word "trick". A trick is something that works for a little while, but then it gets figured out, overused, and finally loses its effectiveness.
Instead, I recommend using what's worked before email marketing even existed and what will exist long after email marketing has died - be helpful, be sincere, be relevant.
The only reason someone buys something is because they have a problem and they believe their purchase will solve that problem.
By being helpful and guiding an individual into solving their problem - you will be successful.
Of course this help MUST be sincere. You must truly want to help that individual and most of the time people can tell if someone is sincere or not ... unless you're incredibly sneaky, in which case you can sell snake oil, but there's probably a special place in Hell for people who use this ability for evil.
Finally, you must be relevant to that person's needs. Let's say you're trying to sell a lawn cutting service - you may be truly helpful and sincere, but if the person lives in an apartment ... what you have to offer will not be relevant to them.
As long as you cover those three aspects (helpful, sincere, relevant) in any and all of your marketing efforts - you will be successful.