What you do is replace the particles with a alpha-textured mesh. So you got your mesh with the particles right? Make a plane with an alpha texture of a blade of grass. Select the plane, shift select the particle mesh and Alt+P to parent. Now select the particle mesh and hit dupliverts. Now all those particles will be replaced with the blade of grass textured plane.
This is a bit of a pain to get to look good, but fortunately RipSting has a brilliant script called Fiber Generator that makes the best grasss I’ve seen to date. Get it here and go to the Fiber Working Thread if you have troubles with it.