If you believe your dog has been bitten by a snake check them over for bite marks, (there may be more than one bite wound). Try to keep your dog (and yourself) calm and your dog as still as possible, movement will help circulate the toxins around the body faster. To reduce the spread of the toxins use an ice pack on the area, this will also reduce the swelling.
Do not attempt to cut into the area of the bite or attempt to suck out the venom.
Try to identify the snake by colour, size and markings.
Seek veterinary attention right away.