you are definitely going to want to tuck your shoulder blades as far back as possible. try to bring them together. it will give you stability by gripping the bench with your shoulders and shorten the stroke of your bench by quite a few inches.
There are many different set ups for bench, so I am not going to tell the way you should do it exactly. But, I myself use a metal militia set up for my bench and feel it has worked great for me.
if you are interested you can read this article and it will give you a good idea.
http://www.metalmilitia.net/index_files/Page392.htmbut again, there are many other set up techniques, this one works for me.