Always be polite and complete in your abuse reporting. A good abuse email should include a complete copy of the abusive email, with the full transmission headers . It should then include a brief statement explaining the nature of the abuse. You should also, include a valid return address so that the abuse handler can contact you if necessary. Another important part of abuse reporting is getting the email to the proper person.
Many companies have a department dedicated completely to dealing with abuse issues.
If the originating network of the abuse has such a department the email should be sent ONLY to that department. Sending a copy of the complaint to any and every email address you can find related to the site accomplishes very little. If you do not know the proper abuse department email, some good guesses are abuse and postmaster at the particular domain.