Simply don't do it.  Spam filters look for this and you will definitely get yourself in trouble if you do this with your emails.

Do not get caught using subject lines with spaces, extra exclamation points, dates, name or email address of recipients and especially the word "free".  There are many others but these are the main ones to avoid.  Use common sense when creating your subject and look to emails sent by reputable organizations for examples of well written subjects.

Similar to the subject line, in the body of the email avoid spam type catch phrases like "you have just won",  "!!!!!!", "weird non-alpha numeric symbols "!@@#@!?", free, once in a lifetime opportunity, etc.  Just use common sense, if you are not sending spam, then this should not be a problem.

Elastic Email supports attachments but avoid using them unless you absolutely require it.  Attachments are a common source for viruses and in combination with other reputation scores can get your email sent to the junk folder faster then anything else.

Send your emails in plain text if possible.  HTML gives the email so much more flair, but can also cause you to wind up in the junk folder.  If you use HTML, make sure it is valid.  Run the HTML through one of many HTML validators on the internet to check it's form.  Avoid using WYSIWYG editors such as MS Word which add so much extra HTML that causes the email to potentially be flagged.  If you use HTML, make sure you also send a text version so that the recipients software doesn't have to try and decode it to text.

Do not change your font color from black.  Black is the most readable and any other color alteration can cause you issues as many spam emails use big bold red colors to try and attract you to read the message.  Keep your background white and use black as much as possible.  Change your background to hot pink and you will probably never see your email again!

Images are fine but use them sparingly.  Do not replace good old fashion text with images.  Use images for images and text for text.  One image emails with everything in a jpg file will most certainly end up in the junk folder.  Keep your image size small as well.

Make sure your emails have a working unsubscribe link that is very viewable.  Do not make it 7 point font at the bottom of your message in an off-white font.  Make it visible, large and clear.  It is much better to be unsubscribed then marked as spam.  Make sure you follow the CAN-SPAM guidelines found here http://business.ftc.gov/documents/bus61-can-spam-act-compliance-guide-business.