You could try essentially squeezing the bar together while you bench, almost like you're doing a fly-type motion while benching. Try to bring your hands together and compress the bar throughout the entire rep (up and down). Also do slower negatives here (while squeezing the entire time), and don't hold a lock out (maybe even stop a hair short of locking out). Even with a light weight, I find it really emphasizes my chest and hits it hard. I wouldn't bench like this all the time, but it may be able to help you learn how to really feel and emphasize your chest when benching. Also, what is your grip like? I find a relatively wide grip works my chest well.
On a related note, I do a similar-but-opposite thing for a lot of back training. When I want to focus on pulling with my lats (as opposed to using a lot of biceps) I use a hook/thumbless grip, and focus on pulling more from the elbows than the hands. Also, opposite to squeezing the bar for chest, focus on pulling/ripping the bar apart or outwards when doing pull ups. I find that it minimizes bicep involvement much in the same way that squeezing the bar while benching minimizes tricep involvement for me. Again, it's not something I do all the time, but I found it is a good change of pace, and carries over into normal lifts, helping me to focus on working the target muscles.