If your aims to break into journalism are rather narrow-focused - like to win The New York Times cartooon caption contest - you'll like this great article from Slate.But there are some handy tips in here for general freelance writing, such as:

• Send your submission to the right person. Know exactly who's going to read your pitch, and send it addressed to them. 
•  Know how the editor thinks. You may have the best idea in the world, but if it doesn't mesh with the editor's ideas or the publication's personality, you've got a dud on your hands.
• Google the competition. Find out who else is writing for the publication you want to break into, and figure out how they've done it. Then see how you can emulate their success. 

Good luck!