Contacting your Representatives - both House and Senate (both State and Federal) - does and WILL make a difference. It has been said that politicians view one letter as representing approximately 100 constituents. One phone call represents about 10 constituents. What you say and how you say it significantly affects the impact your communication will have.
Any letter or phone call resembling an angry rant or conveying a "You better not do so and so..." attitude will do NO good. Office staff will pay attention to thoughtful, respectful, and informative communications that clearly and simply state your position on an issue. "For" and "Against" scores are kept and staff screeners will pass the best of the letters on to the elected official.
One last suggestion... multi-format is better. A phone call, followed up by a letter, and then reinforced with an email or a fax is better than an email by itself. These multiple communications should not be mere duplicates of one another. Each should be slightly different in content or approach. This will provide your Representative's office with different perspectives on the same issue. It may also be the case that different staff members are assigned to the various streams of correspondence - one for the phone, one for email and social media, and someone else for letters and faxes. This means your message will be reviewed by more than one staff member and will be even more powerful.
Choose your words as carefully as you choose your ammo and fire away! Reload often and you WILL make a difference.