Using these techniques, you can minimise your risk of your email marketing campaigns ending up in the junk mail folder:

Your design should have a good balance of text to images. If you send your campaign as a single embedded jpeg then this is 100% image and you are more than likely to end up in the junk mail file.
You can run an online test such as http://www.emailspamtest.com/#
You can avoid ‘junk’ or ‘spam’ keywords – get a full list of common ones here
You can ask key customers to add your domain name to their ‘safe’ list