What recipes have you tried? Have you checked out the sugar scrub recipes at the SoapQueen's blog? She has several tutorials:
I've tried these and they always come out perfect, eventhough I still love to buy Naiad Soap Arts' (guest blogger for the tutorials) sugar cubes and scrubs.
Like it's explained under the comments section(s) of these tutorials though, the type of M&P base, oils and fragrance oils used can affect how these come out together with getting the right temperature and consistency for the soap before adding/mixing in your sugar. If your cubes are coming out too hard, you might need to adjust the oils, M&P base and/or sugar amounts until you get the consistency you want. What might be perfect for me might be too hard or crumbly for you. One quick way to test if you like the consistency of your final product is to scoop out a small portion with a spoon or popsicle/wooden stick (not a toothpick - the sample will be too small) and stick it in the freezer to harden/set up then check if you like the result. HTH!