When you arch your back and lay on the bench, make sure to keep your upper back tight as well, as if pinching your scapulae together. Also, try to tuck your elbows in to your sides: don't let them flair out. When I do both of these, the weight goes up a bit better.
Also, everything that has been said by CallMeKing, Rick, and BTB is critical as well.
For triceps work, I like dips and floor presses. The latter you can do with a barbell or a set of dumbbells. I did them yesterday for the first time in a while, and my triceps are very, very sore.