Correct form for bench?

I've been training with a trainer at 24 hr fitness who has said to make sure my back is flat when doing any kind of bench work, incline or decline....barbell press, dumbell press, flies, etc... and if it's not flat, then it puts too much stress on your lower back. With my recent bb.com order, I chose a dvd focusing on back and chest. On the dvd, the guy says you should arch your back when doing benches.

Which way is correct and why is it correct, why is the other way not correct?

Thanks,
JB

"...take whatever you want it's your body. Just don't come crying back because you've grown a set of tits and need to take viagra to get a hard on." ------- odw777

Its fine, I hurt my shoulders more often when I didn't ark my back. Plus you get a way better stretch.

17 years old
180 pounds

dirty bulk= fail
X-repper forever

On the race to 190 and 17 inch arms

Arch your back, but keep your butt on the bench.

you want to arch your back and kind of push your upper ribs outwards.  this will do the most for you

sorry, but trainers at the gym don't know as much as you think they do.  i'll go to the gym and see the trainers tell the trainees to do something, and it'll be completely wrong.  it's sad

I'm a trainer and know that a lot of trainers don't know a whole lot. I'm not saying you have a bad one but you need some arch in your back. Now it could be that you were arching far too much and he told you to keep your back flat to correct the problem and you're just wording it here in a way that makes him sound wrong. Butt on bench, upper back on bench.

5'6" 125/ Bench: 115 / Front squat: 90 / Deadlift: 205 / Pullups: 10

stick your chest out, arch your back, and pull your shoulders back(squeeze shoulder blades), flex your chest at the top of the lift, do not lock out your arms
 
this keeps the shoulders/triceps from doing most of the work when benching

Thanks y'all. 
 
Ripped chick, what you described could be a reason.  My trainer really tries to drive home the importance of always using correct form.  My shoulders used to (sometimes still do when I'm getting tired) elevate and come forward a lot during some exercises.  My form on several exercises was all screwed up.  So it is possible that I was arching way too much before.  Never thought of that! 
 
JB

"...take whatever you want it's your body. Just don't come crying back because you've grown a set of tits and need to take viagra to get a hard on." ------- odw777