Yes, my 3 DCs gained places at a grammar school without professional tutoring (from a state primary school which gave no advice or help),
I 'prepared' them by buying papers from WHSmith and going through them, showing them ways to approach the problems, sometimes having to explain words they had not come across, doing timed tests etc.
I think once you have established they can actually do the tests (i.e if they really find it hard, then prob not a good idea to enter for a GS test in the first place), then the main thing to do is work on the speed at which they can do the test papers, telling them not to spend too long on questions they can't do etc.
In our area, for the super selective school, you are looking at getting very high results on the practice tests in order to be in with a chance, but you can work up to these scores. However, I do think in the real thing they score lower, IYSWIM. Certainly my youngest claims he left quite a few out on the day, but still got a place :)