Practice all the time. If you want to be a good comic, you have to get out and perform. Go to open mic nights in your community, try karaoke or audition for an acting role. Join a public speaking group such as Toastmasters, a debate team or an improvisation group, or even just make a speech or crack a few jokes at weddings and parties. As an aspiring stand-up comedian, it's most important to get out of your comfort zone and try out your routine.
Write strong, relatable material. The adage "write what you know" works just as well for the fledgling comic as it does for the aspiring writer. Woody Allen's early career in stand-up was based on personal material. Some of the best material working comedians have comes from their wry and astute observations of their day-to-day lives. Crowds will laugh with you if they can relate to you. Being alienating can work too, as it does for comics like Stephen Wright. There is no magic recipe for universal appeal, but identifiable material in a contemporary setting is a good starting point.
Get your routine down. This goes hand-in-hand with practicing, and a good comedian always knows his own material. Don't be so rigid in your delivery, however, that you don't allow room for spontaneity and improvisation. The best way to go off on a Robin Williams-style tangent and add zip to your material is to know it inside and out. The best way to stray from the script is first to know that script to the letter.
Accept that some performances you do will not work out. Just as no two crowds will always laugh at the same jokes, not every night is going to have the audience rolling in the aisles. When the set that last night toasted the town tonight falls flat, use it as a learning experience. This is how a performer learns some humility and also begins to get a feel for reading the crowd.
Develop a thick skin. Just as many comedians build a schtick around sometimes confrontational or obnoxious antics and sarcasm, be prepared to be heckled from time to time. However you choose to respond to booing and taunting, it is always best not to let it taint your performance.
Make your routine appropriate whenever possible. Being crass can be very funny, but if it generates feelings of discomfort or potential hate-mongering, you risk damaging your career. If your material is heavily sexist, racist, homophobic, or hateful, even if ironically so, be prepared to face the consequences. Comics such as Michael Richards, Tracy Morgan and Andrew Dice Clay have damaged their credibility for being too risqué and scathing. There is a fine art to being a shock comic and not everyone can master it.
Be supportive of your peers and go to lots of shows. There is a good chance that your city has a thriving community of comics and improv groups performing regularly; seek them out. Network and socialize with these other comics and groups and learn and grow with them. This a good way to meet like-minded artists, test material, get some valuable critiquing, and build an audience.
Study the greats and emulate them in little ways. The list of genius comics is far-reaching and expansive -- among them Woody Allen, George Carlin, Groucho Marx, Eddie Murphy, Phyllis Diller and Joan Rivers. Watch, read about and familiarize yourself with the techniques of your favorite comics to gain insight that you can apply to your niche.