Basic Time Travel Etiquette
It is generally assumed that, when traversing the fourth dimension, you become privy to the true nature of causality, and that it will be in one of the following three formats:
The timestream can never be altered. See: Slaughterhouse Five, Twelve Monkeys, Oedipus Rex.
Changing the past creates an alernate, equally valid timeline. See: Sliders, arguably Terminator.
Changing the past results in an altered future. May or may not jeopardize all of reality. See: Back to the Future, A Sound of Thunder, etc.
Although most armchair physicists agree that A and B are the more likely scenarios, if you are uncertain which model of universe you inhabit, assume you are in a model C, and changes you make to the past can have serious consequences. In any model of universe, except in case of horrific robot apocalypse, it is considered very rude to intentionally change the past, and downright unethical to do so for personal gain.
When first spontaneously catapulted into another era, a simple glance at a nearby news vendor (whether robotic or lovable suspendered urchin) should suffice to pinpoint the date. There is no need to walk up to a stranger, shake them by the shoulders, and say "The year, man! What YEAR is it?!".
When visiting another era - whether future or past - remember, you are a visitor. Avoid bragging about your civilization's accomplishments or turning up your nose at your ancestors' or descendants' habits. When in Rome, do as the Romans do! (Obviously, exceptions to this rule can be made for flagrantly immoral behaviour such as kidnapping, cannibalism, or bizarre ritualistic cyborg gangrape.)
When dealing with duplicates of yourself, immediately change your appearance to prevent confusion - pretending to be your younger self can be amusing, but those types of practical jokes inevitably turn sour. Whichever version of you is visiting from elsewhen (whether older or younger) is the one who has the responsibility to make the change. Generally, you would cut your hair shorter and start going by your middle name - that's why your parents gave you one! If you have more duplicates than middle names, refer to yourselves by either the Greek alphabet or the military callsigns (whichever you have memorized), with Alpha being the youngest. Consistency, here, is key - if the group of you changes eras, stay with the same names. The whole point is to avoid confusion, both for others and amongst yourselves.
When you and your duplicate are attempting to convince some third party of the veracity of your claims, remember: moles and skin tags are your best friends! They're hard to fake, stay in one place over the course of your whole life, and even twins or clones wouldn't have moles that match!
Always remember that most money is stamped with the date of its manufacture! Check the dates on your coins and bills before you attempt to spend them. If you're travelling more than twenty years in either direction, bring pawnable gold - even in the far future, jewelry is unlikely to become completely worthless. Also, future coinage is a good way to prove that you've come from the future - few people would go to the trouble of counterfeiting an unspendable hoax penny.
Hygiene is essential! Remember, there may be new diseases in the future against which you are not vaccinated, and the past has things like polio and the bubonic plague. Wash your hands, both for your sake and for theirs!
If at all possible, avoid cross-era romantic relationships - with few exceptions, they're usually not going to work out. As regards sexual intercourse with older or younger copies of yourself, it's incest in branching or changeable timelines, and masturbation in inevitable timelines. In either case, yes, it is gay.
If you're in the past, and you need to write a message to the present or future that shouldn't be read until the just the right moment, consider using a Vigenère cipher (look it up!) with some piece of current events trivia as the translation key. Vigenère ciphers are simple enough to be able to code and decode them in your head, yet as long as you keep your message short, leave out spaces between words, and the length of the key remains unknown, the cipher should prove effectively unbreakable.
Have a time travel password!
Mental passwords are useful not only in time travel scenarios, but for any situation in which you need to send a message to yourself (See: Memento). Ideally, you want to have two - the real password and the decoy password.
Don't ever write your passwords down, don't ever speak them aloud, but make sure to commit them to memory anyway. When asking for your time travel password, think about your decoy password - in theory, this will prevent you from being scammed by telepaths (If you're dealing with a hostile telepath that can see past your surface thoughts to dredge up a specific piece of information from your memory, you're screwed regardless).
Now, mental passwords only work as long as you can trust yourself to follow through. You need to be prepared to lock and load if someone you don't recognize walks in the door and says "Hey, your future self says you have to assassinate the mayor. Trouserfish.".
Obviously, when sending a text message to yourself, you can Vigenère it with your time travel password as the key.
Also, don't kill Hitler, even if it really seems like a good idea. Just trust me on this one.