OK this might not be revolutionary but in the spirit of the credit crunch I thought there's no harm in sharing.
I bought a pack of Sainsburys basic nappies (98p I think) and they aren't great. However, whenever DD has been at nursery there is that couple of hours between getting home and bedtime. I've stuck one of these basic ones on just to cover that couple of hours (if that). It hasn't got time to leak, and if it's just one wee I don't feel it's so wasteful binning it after such a short wear.
Probably only saves about a pound a month but I think it all helps!