Your first and best bet, especially if time is of the essence, is a phone call. You are unlikely to be able to speak directly to your Federal or State representatives. You are much more likely to be speaking to a staffer. On the local level, depending how things are organized in your area, you may speak directly to your representative or to a staffer. If a vote is imminent, the staffer will make note of your feelings pro or con, collate them with other calls, and deliver a summary to your legislator before the vote. Sometimes you can give a few quick talking points, but if you have left your contact to just before the vote these will not have much impact.
When you have time, you will have the most impact with an old-fashioned letter. Keep it polite and concise. You will almost always get a response, usually within about 6 weeks. In a lot of cases, it will be obvious from the response that no one actually read your letter, so have a slightly less polite (but still polite – you can express your irritation, however) letter ready to fire off by return mail. This is also probably worth an irritated phone call to a staffer.
When you don’t have time for a snail mail letter, send your letter in an email. But just because it is an email does not mean it should be informal. It should still be formal and respectful.
Find Your Lawmakers
http://the-cavalry-group.rallycongress.com/congress/ – This will give you federal and state with website links for the federal
http://hq-salsa.wiredforchange.com/o/5950/getLocal.jsp – This will give you federal, state and local legislators, mostly phone numbers but some email addresses.
https://www.usa.gov/elected-officials – Federal, state and local levels
How to Write the Letter
http://www.artofmanliness.com/2011/06/17/how-to-write-your-congressman/
http://usgovinfo.about.com/od/uscongress/a/letterscongress.htm