1. Do not forward crap: if it is noteworthy to send to others, remove the forward form the subject line, clean up the text. If it is something you want to talk about that is trivial, post it on your blog, Facebook or twitter, email is for more of a direct communication.
2. Do not put an image in your signature: images in your signature are super 90's geekness, not to mention it takes up valuable space and bandwidth (especially for mobile devices). When someone must look for a file from you, they will see an attachment on every mail from you. it screams newebie and begs for excommunication.
3. Use BCC not CC. I don't want other people to have my email address so the next time they click on a rogue attachment I am emailed. use the BCC option when sending group emails, UNLESS it is imperative that everyone see who is in the "group" being emailed, for example an email to a few co-workers about a group picnic, who is going to bring what.
4. Do not send things via a website. If you see something cool on the web, copy the link in the browser and paste it into the email you want to share with someone. If you use the websites "send to a friend" it gives the website owners your friends email address"