I agree with you Cod on telling them to go, not asking, and on being prepared for loads of messes.
But I have a golden rule too which some mums might like to remember (I have potty trained three children so far) and that is this:
when you get to three wet pants in any one day, give up for that day and start again the next day. I started that rule when I was potty training my first and the wet pants were moreorless continuous. I decided it was necessary for my piece of mind! (and none of mine took more than a couple of weeks from nappies to completely dry pants- day and night)