Contact the author or publisher of the material if it is going to be commercially beneficial to you -- such as being used on a website to promote sales or in a pamphlet promoting your business. Ask for express permission in writing and keep the response -- whether negative or positive -- in your records for documentation.
Credit the author for the material whether or not you are using it for commercial or educational purposes. If you are not using it to benefit monetarily (for example, you are using it for an educational purpose), then do not worry about contacting the author. Such usage falls under the Fair Use section of copyright law.
Consult with the United States Copyright Office guidelines if you are not sure whether or not you should be using copyrighted material without permission. Use a copyright material checklist such as the one found at the University of Michigan website to make sure that you have a right to use the material without consulting the author.
Use a service that helps you check for copyright permission electronically. Although these services are fee-based, they can save you the time and effort of writing and keeping up with a letter through the mail. The University of Texas suggests using the Copyright Clearance Center for all your electronic verification needs.