Well... 150mm, short distance, f/2.8... you are going to get a very narrow depth of field with that.
As "stever" pointed out, I would first try to close the diaphragm, but even at f/11 or f/16 you will not get a lot of DOF.
I have done this a fair amount and know pretty much with this setup that you are right, dof is very limiting to say the least. I do not think I have ever had more then 3/4" to 1" dof with this setup regardless if I'm at f/2.8 or f 11.
Just seems to be more a function of the 500d then aperature settings.