I simply put my wet/dry shop vac hose in the spa, and turn the vacuum on. Once the water starts flowing, I pull the hose off the shop vac and let the siphon do the rest. If I keep the wand on the end of the hose, and I can then "vacuum" the sand off the bottom while it drains. The 2" shop vac hose has it empty in less than 5 minutes. When the siphon stops, I can connect the hose back up to the shop vac and finish vacuuming the seats and other areas where the water gets trapped.
I bought a 6ft extension hose to things easier, and also taped the joints with black electrical tape to keep it from sucking air.
On a water changing side note, a friend of mine puts his pool cleaning hose over a spa jet while the pump is running, and his wife de-ices the deck with the other end. Seemed to me like a good use for 100'f water that would just have been dumped on the lawn any ways.