As far as I know, a lot of DDoS attacks use UDP amplification, which can be prevented if every ISP implements BCP 38; i.e. drop UDP traffic at the edge of their network that has a source that cannot have come from within their network.
EDIT: To clarify, this won't stop layer-7 based DDoS attacks, or anything that uses TCP (like SYN flooding). Just UDP amplification.
EDIT: To clarify, this won't stop layer-7 based DDoS attacks, or anything that uses TCP (like SYN flooding). Just UDP amplification.