Always Do Your Research.

No man’s knowledge here can go beyond his experience.

John Locke (1632-1704)

Research, research, research. I can’t say it enough. No one wants to look stupid in front of his or her audience. I’ve seen brilliant people make very embarrassing mistakes. And nowadays especially never trust one source for your research, especially if you are looking on the Internet. Read multiple sources to gain a well-rounded expertise about whatever you are writing. Never take anything for granted. If you use slang in fiction, make sure the term was used during that period. If you talk about a specific gun (which, of course, you’d better, because you want as much detail as possible) in a cowboy story, make darn sure it was actually used by cowboys then.