There are essentially two things to know, neither of which are really that unusual but are surprising in how many people don't act on them.
#1 is printing your boarding pass in advance. Some people use the kiosks at the airport, which is better than standing in line. But the kiosks still require you to stop by the ticketing level of the main part of the airport. With a home-printed boarding pass, you can go straight to seccurity, which is the floor below ticketing. (Not to mention the fact that you can often get a better seat by taking your seating into your own hands 24 hours before your flight -- I just checked in online for my United flight and was able to move from a middle seat to a window.)
#2 is for frequent fliers. If you have 'status' on any airline, you can go to the short line for security check-in. It's worth noting that until recently this was for status of any level; now, however, you need second level status (e.g., Premier Exec for United, not just Premier) to use the short line. Further worth noting, I got in line in the short line just to see if they would really make me leave for just being Premier -- the TSA agent didn't even check my status. (Make sure you compare the length of the 'elite' security line with the regular security line; oftentimes the multiple server setup of the regular line gets you through faster than the elite line, especially in major hub airports where more and more people have status.
Some people walk from the terminal to the A Councourse to go through security up there. This is the best way if you're on Frontier (which comes/goes from A Concourse). That way you don't have to ride the train. Sometimes this can be faster even if you have to then go downstairs and take the train to B or C Concourse, but rarely.