iso doesn't effect dof.
i was going to recommend going to a 35mm lens, on that t3i, that's = ~50mm, so you will get a deeper dof at the same aperture.
but there are many variables, how far you are from the scene, how far the subject is from the background, your focal length and f-stop.
so i would suggest doing a bit of the math on a dof calculator such as
http://www.dofmaster.com/dofjs.html and plugging in your specifics to see what your dof is, then make adjustments to any of the variables you can to create the perfect dof.