EDIT: Featured on the instructables and lifehacker homepage, awesome! Just to clear some things up, the evaporative cooling doesn't work in humid environments but you can still use ice packs. And it's probably not as effective as just pouring water directly on your dog, but you will get the same effect without actually wetting your dog (and the wet dog smell), but anyways enjoy!
Need to keep your pooch cool for long walks on hot sunny days? Well look no further for this multipurpose vest will fit the bill (and your dog)!
The basic premise for the "cooling" part is you soak the the upper layer and as it evaporates it will draw heat from the lower layer with a waterproof layer in between to keep your dog dry. And also since it has side pockets you could throw some ice cubes in there too which will melt and then evaporate. Or you could just put flexible freezer gel pads or what not in there (or bags of frozen peas for that matter).
Old worn out cargo pants
Pins (safety or otherwise)
Step 1: Cut out inside seam
Cut out the inside seam as indicated in the picture.