Say hello. When you're walking past someone, even a stranger, try to acknowledge their presence with simple "hello" or "hi" or even just a wave or a nod in their direction.
Ask people how they are doing. Take the time to ask someone how things are going in their lives, without being nosy or intrusive. If they seem resistant to talking, just let them know that you're always around to talk to, and that you want them to be alright.
Be a good listener. Listen when other people are talking to you. It isn't nice to just ignore other peoples' opinions and stories. If you find that someone is becoming rude or pushy, acknowledge their opinion, issue a compliment ("Having your own set of values and beliefs is pretty admirable") and excuse yourself politely ("I'm sorry, I've got to go get the groceries so I can meet my husband/wife when they get home.").
Be positive. Don't be negative or critical. Keep looking for the positive in any given situation.
Be humble. The key to being nice is remembering that you are not "better" than someone else. You're an individual, but everybody has their struggles, and being nice to one another makes life better for everyone.
Offer to help. If you see someone struggling or doing anything, offer to help, even doing something as simple as carrying a bag of groceries. You can also be nice to the community and the world by volunteering.
Be sincere. Don't be nice as a means to an end. If you just want to be nice so that you can gain preferential treatment, it's quite the opposite of being nice--it's deceptive, shallow and cruel. Be nice because you want to look back on your life and know that you were a nice person, no matter what.
Always remember, The Golden Rule: Treat others the way you want to be treated.
Assume the best about people. Most people don't mean to insult or offend others most of the time. Unless it's overt, assume the slight was accidental.
Bad language, gossiping, whining, selfishness and greediness don't mix with being nice.
Hold the door open for someone who is approaching the door the same time you are. Allow them to go in ahead of you. Do the same if you are leaving a store, hold the door open wide, and allow who ever is leaving, leave before you.
If you are sitting, and you see an older person, or a pregnant, or ill person standing, offer them your seat.
It may seem a bit awkward for guys to smile, especially at other male strangers. If that is the case, then simply ofter a casual "hey man", or "hi" or "how's it going?". It works just as well as a smile, and can be more comfortable.
If you find yourself thinking poorly about someone, don't worry; you're not a terrible person. But try to catch yourself doing it, and think of something nice about that person instead. It'll help you look at people more positively, and you'll quickly break the habit of seeing the worst in someone.
Don't laugh at other people's mistakes and don't point out their faults too harshly.
Be optimistic about everything.
While being nice, do not be a total pushover. Compromise is good, but expect to be treated fairly. Don't be afraid to stand up for what is right and do not hesitate to defend someone. If you find that you're being considerate of someone's time but they are not being considerate of yours, bow out as respectfully as you can and make yourself scarce.
Members of the opposite sex can sometimes misinterpret niceness as a signal that you're attracted to them, or even as flirting. Learn how to gently and subtly let someone know that you're just being nice, which by the way is not the easiest thing to do, but it will prevent a lot of miscommunication. Do not go over the top with being nice because people will twig that you are only doing it for yourself!