Visits are important on key issues when your legislators' votes really count. It is more difficult to say no in person than over the phone, or in a letter! They also serve to keep you in mind as a source of information if your legislators can put a specific issue together with your name and face.
Call their office or drop them a note before your visit. Let them know specifically what you wish to speak to them about, and offer them two or three alternative times you are available.
Address legislators as "Representative" or "Senator".
Legislators are often between votes and your time with them is limited. Get to the point or you may miss your opportunity.
Be prepared when you arrive. Do your homework in advance so you can speak clearly and concisely on the issue.
Have something in writing to leave behind with the legislator, preferably a one-page fact sheet on your issue and position.
Always thank legislators for their time and express interest in keeping in touch, even if they disagree with your position this time.