1. Do write / call them. They don't know what you don't say. The cardinal rule in politics is that people only contact politicians when they are opposed. Be sure and let your Senator know when you support them. If you are going to write or call them - learn the how the mail works, and how to leave a message (include name, address and phone number). Don't send in a form letter!
2. Don't write Senators other than your own. If you're not a constituent, the Senator will most likely ignore you, unless you have a casework issue and then they will pass the issue on to the appropriate person.
3. Do include facts, sources, references, and how your neighbors / family feel on the issue. One of the hardest things for a Senator to do is to judge how his constituents feel, simply because of rule #1 - most people only write when they are opposed. If you can vouch for your neighborhood or an organization, let the Senator know.
4.Don't write emotional letters. For example, simply saying the Patriot Act is evil and infringes on your civil liberties doesn't work. Read the act, and tell the Senator were you believe the act infringes on your civil liberties and why. If you can point out specific language that you feel is wrong and request action on that language do so, it will help the Senator to know exactly what you what changed.
5. Do contact them in advance. Writing two days before an issue is to be voted on will have little effect. Most likely, if your Senator did not have an opinion beforehand, she / he will have already made her / his mind by then. Bills spend months in commitees (sometimes) so write when the bill is in committee (because even if your Senator is not on the committee, he / she may be able to call in a favor with a Senator who is).
6. Don't swear, use vulgar language, or be crude. I know this seems like the obvious, but a decent amount of constituents don't follow this rule.
7. Do try and visit them during recess. When the Senate is out of session, Senators return to their home states. Almost all Senators hold town meetings or county nights during which it is possible to meet in person with your Senator. These are extremely worthwhile to attend, especially if you are willing to discuss your concerns in a rational manner (to find out about them contact one of your Senator's state offices). If the Senate is in Session and you are comming to DC, call your Senator's office in advance and ask for a tour and to meet with her / him. Many offices do something like a constituent breakfast, when you can go in during the morning and chat with the Senator for a while.
8. Do offer to help. After letting the Senator know your concerns, ask what you can do to help spread the word, educate people, or be of assistance in some way. Please try and educate yourself on how certain parts of the Senate work, like Conference Committees, or holds if you offer to help with legislative issues.
9. Don't rip other parties, political opponents or offer to break ethics rules (duh!). Senators have to be very careful who they criticize, since they may need the support of a friend on the opposing side in order to pass an ammendment or bill.
10. Don't give up. Not all Senators are friendly. There are a few Senators on the Hill with a reputation for being less polite than others (Thankfully these are only a few). Most likely your Senator is not one of these, she / he is simply very busy. Be paitent, and keep trying.