Here is just an example of email latter that I use to exchange link with other websites. Alter the template according to your need.

Email template #1 

Dear Webmaster,

I have visited your website constantly and I think that the website content could benefit our website visitors.

I have already placed a link to your website along with a description at "http://www.you-site-name.com/link-page.htm". If you would like to modify the description of your website or if you have any idea please let me know.

I would appreciate if you could place a link back to my website.

[Your website description]
[URL]

Best regards,

Your Name
[Your-website] [Your email]
[Physical or postal address]
[phone number]

Email template #2

Hi Webmaster,

I have visited your website [their website address] today and I like it. I actually run a website that similar to yours. I was wondering if you woule like to exchange link with me so we can benefit for each other. I have already placed your link in my website along with description here. You can check your link by visiting here [Website link].

I would appreciate if you could place a link back to my website.

[Your website description]
[URL]

Please let me know if you have any questions or comments.

Your Name
[Your-website] [Your email]
[Physical or postal address]
[phone number]