rusty sills

[scoots66]

I've an '03 Nevada 1.8 and have rust coming through (badly) on both sills (just infront of the rear wheels). This i see appears to be quite common and I'm having both replaced and sprayed for a decent £300. The garage have suggested they drill a few small holes out of sight underneath and fit rubber bungs so i can let any further water out.
My question is, how do i stop water/rain/damp from getting down there in the first place? Does it travel down the seat belt housing? Is there a cure ?
thanks...x

[scoots66]

oh !!! cheers, ive looked underneath and didn't see anything too obvious.. will have another look later (altho it's too late now!)
cheers...