Hiking Etiquette
 
Take out what you bring in (food wrappers, etc.).
If you spot trash left by others, grab it and pack it out.  (It's guaranteed to  improve your karma.)
Don't feed the animals.  Human food is not very good for them and can  make them sick.  Also, if they become used to human food, the rangers may have to destroy them (as in the case of bears).  Finally, animals may bite, they may have rabies and they may have fleas that carry diseases.
Leave any flowers you find.  Picking them is not only bad for the environment, you can be fined.
If there are other hikers who are faster than you, step aside and let them pass. 
Go the bathroom away from any streams or lakes. 
If you are going to play music, use earphones.  
If you are a smoker, be extremely careful of the fire danger.  Also, take your used matches and cigarette butts with you when you go. 
Take lots of water:  two bottles is recommended.  If going any distance, make sure that you have enough to get to the next place with potable water or take tablets or a filter to purify creek water.  (You can get giardia by drinking untreated water.)
Take sunscreen, chapstick, good hiking shoes, insect repellent, a hat, sunglasses, camera, a day pack, some food and a jacket or shell in case things turn cold.  Always put insect repellent on your ankles and lower legs to discourage ticks.  Take a poncho if it might rain, take moleskin (and something to cut it with) if you think you might get blisters, and take a flashlight if there is the slightest chance you'll return after dark.