1. Ticking Off Your Email Service Provider or ESP (like Constant Contact or Mail Chimp)
If you've just been putting everyone who ever signed up for your email list or did business with you into the "To" column of an email and sent it out, you've done badly. This is actually against the CAN-SPAM Act, and violates most service provider agreements. So don't do it this way.
2. Not Segmenting Your Email Lists
Every successful email list has segmentation by gender, age group, and whether the person is a past buyer or not. You can also experiment with deeper segmentation, such as by city if you build in several of them, and by how long ago the people have bought from you. The more specific you are, the better you can target your message.
3. Using Images Poorly
Images need to add to your marketing campaigns, not dominate them. Keep in mind that not everyone's email client loads and displays images by default. If all you send is a group of images with fancy CSS coding, that's cool. But a significant percentage of the people you're sending it to won't see it, and probably won't even know what you're talking about. Not to suggest that plain text is your only option, but seeing your marketing emails in plain text should at least be an option.
4. Not Allowing People to Unsubscribe
Your unsubscribe button doesn't have to be fancy or draw the eye, but it does have to be there. By law, if you don't have one, people have every right to blacklist you with the "Spam" button. You might still build buildings, but you won't build your email credibility this way.
5. Getting Turned Into Spam
Once you're in the junk email folder, it's hard to get out. Don't include attachments, do have an opt-out button, don't overly rely on images, and make sure you allow your emails to be viewed as plain text.