If you have a collection of just a few really common emails that are sent repeatedly to numerous customers and prospects, you are likely using the email templates functionality. If you get complaints about how long it takes to pick the template from the pop-up window, I have a solution for you…

You can create a “Send an Email” button that pre-populates a specific template into the form. This means you could have your existing “Send an Email” button plus a “Send Reminder Email” and have the form automatically populated with the “reminder” template.

To start, pick a template that you would like to use. Navigate to Setup > Communication Templates > Email Templates and then open the template you would like to use. In the URL / Address bar locate the Salesforce.com ID of the template. In case you’re not familiar with the Salesforce.com IDs, it should be a 15 character ID that starts “00″ and ends with some alpha-numeric characters. Here is one of my template URLs with the Id underlined for illustration:
https://na4.salesforce.com/00D40000000yz5a?setupid=CommunicationTemplatesEmail

Once you have the ID, go to a Lead or Contact and click your old-fashioned “Send an Email” button:

Once the email composition page loads, copy the full URL of the page. It should look something like this:

https://na4.salesforce.com/email/author/emailauthor.jsp?p2_lkid=00Q6000000zzTI2&rtype=00Q&retURL=%2F00Q9000000DytI2

With both the template ID and the URL for the “Send an Email” page, we are ready to create our custom button. Navigate to Setup > Customize > Activities > Task Buttons and Links and press the “New” button to… create a new button. :)

The new button needs to be created specifically for the screen we will place it. In this example we will use a lead. Paste your “Send an Email” URL that you acquired in the previous step. Now, replace the two Salesforce.com IDs in the url with the string {Lead.ID}. When the button is placed on the screen, this will make it always link back to the lead screen from which you clicked the button. Append the string “&template=” and then the ID of your email template to the end of the URL. This will now default the button to that template which you picked before.
The full URL in my case now looks like this:

https://na4.salesforce.com/email/author/emailauthor.jsp?p2_lkid={!Lead.Id}&rtype=00Q&retURL=%2F{!Lead.Id}&template_id=00D40000000yz5a

Press the “Save” button to save your work and create your shiny new button.

Navigate to Setup > Customize > Leads > Page Layouts to add this new button to a page your user can see. Edit the layout where you want it placed. Click to select the related list at the bottom for “Activity History” and then click “Edit Properties” button. This will give you a pop-up configuration window where you can add your button to the header of that related list. At the bottom of the pop-up window is the section you need:

Move your new button into the “Selected Buttons” list, click “OK” on this window, and then save your page layout. You should have a button now on your Lead page that auto-populates with the template you desire.