What I have done personally in the past is take about a month or 2 off from doing normal flat bench and try a few different lifts.
Heavy dumbbell flys: go down so you feel a good stretch at the bottom of each rep the extra stretch will help your muscle grow. I like to do these as the last part of a chest routine so I get the stretch at the end of the workout.
Explosive pushups: regular pushups, clap pushups, triangle clap pushups, fist to clap pushups etc. Go down slow and make sure you explode on the way up as hard as possible each rep.
Cable crossovers: if you have them at your gym, use them, they are fun and hit your chest from a diff angle.
Incline/Decline bench: both incline and decline focus more on your chest than the flat bench will try going heavy on these, do incline one week and decline the next.
Vince Gironda Dips: Check this out, they kinda feel weird at first but they are the best dips for hitting your chest.
http://www.ironguru.com/vince-gironda-dips
Also you use a lot of your front deltoids and triceps when you do the flat bench press so start training those parts heavy aswell.
For front deltoids try the deltoid front raise:
http://genxxl.com/workout/dumbbell-front-raise-best-deltoid-exercise/
For triceps (VERY important to increase bench): Do skullcrushers, close diamond/triangle clap pushups, normal dips focusing on your tris, tricep cable extensions, and close grip benching (go slow at first so you dont drop the bar to one side).
On top of that make sure you are squatting and deadlifting. Squats and deadlifts make your entire body grow, if you are not doing them, start doing them!