A few basics to remember:
Every email has a purpose.
Every sales email has a call to action.
Don't be like the eager new sales person and rush the sale in your email.
Your call to action appears after your copy has made your reader ready for it.
Up until the point of the call to action, your copy must show your prospect the benefits of taking action. You can do this by telling a story - a recent success, a customer testimonial, a mini case study, a snippet from a conversation.
Honestly, if you ask too early they will ask: "Why should I?" and you’ve lost them.
In most cases, plan to place your call to action near the end of your copy...before the signature line and then again in the PS.
Here's an example of a sales email I wrote for a boudoir photographer. I told a story before asking the reader to take action (click on a link):