When you see a dog coming up to chase, instead of just hosing them with plain water, swish a mouthfull of water around like you're rinsing at the Dentist and spit on the ground in front of them.

The smell of your mouth is almost as irresistable to a dog as the smell of your, um, other orifice. (For those of you that know dogs) I haven't come across a dog yet that can pass up a puddle of fresh spit, while several will run right through plain tap water.