i use a main feed the day before and then give them a tin of tuna, or an egg 2 hrs before we go out, just for a protien boost. My father used to race whippets and to clear them out before a race he would place a match stick 3/4 of its length in the dogs arse. This encourages the dog to empty out before racing. i seen it as a kid and it works, weather it takes time off i dont know but it makes sense to me.