The CAN-SPAM Act of 2003 was a great start to lowering the amount of spam messages people received. It was also very easy to comply with and most people only needed to add a few simple things to comply. This only applies to commercial email, of course, not personal emails.
The obvious ways are to just be honest. Don’t use fake email headers, use a subject line that actually states what the email is about and to state if it is indeed an advertisement or not. Also, you must provide information about yourself in the form of a mailing address. It isn’t required to be your home address, but a valid address you can receive mail at (like a PO Box) is enough.
The other requirement is to provide opt-out methods that actually work. It needs to be stated clearly how a person can opt-out and then requests to opt-out should be processed quickly. Most of the time an unsubscribe link that automatically removes someone from an email list is all that is needed.
And lastly, if you hire someone to help you with marketing, monitor what they are doing. Even if you aren’t spamming your customers, if you hire someone who does, you are just as responsible for their actions.