When treating a snake bite what should you do with the affected limb to decrease the spread of venom?

Keep the affected limb below the level of the heart to decrease the spread of venom.

Call an ambulance and go to hospital quickly.

Make sure not to panic, because in some snake bites (hemotoxic or hemorrhagic venom) this can make blood flow faster increasing the spread of venom. Stay calm.

Fact.. You are more likely to avoid being bitten directly if you wear long pants and ankle height boots when walking in areas suspected of snakes.
Keep it still, ice it, keep it slightly lower than the rest of the body. Then move them as fast as you can to the nearest hospital.
Keep it lower than the victims heart - and apply a pressure bandage to the bite area. This will slow the spread of venom through the bloodstream. Do NOT us a tourniquet - as when it's released the body will get a massive dose of venom.
Apply a pressure bandage to the bite area - NEVER use a tourniquet ! Immobilize the limb to slow down bloodflow, and thus the spread of venom through the body.