In all our years of working on cisterns, we've seen, tried, and heard about many different ways of cleaning out a rain cistern or spring tank. Some ways are more, shall we say, "creative" than others, but only a handful have proven themselves to be effective and safe, which is exactly the metric we strive for in conducing our business.
Below is a link to a video tutorial we did that goes over our time-tested technique. Please note that you may need the following tools for cleaning out your own tank:
*Sterilized ladder (wash down with chlorine solution)
*Sterilized boots or hip wadders
*Sterilized pump (we prefer a 2" submersible trash pump, such as ones made by Gorman Rupp)
*Discharge hose for pump
*Food-safe, sterile drum or barrel
*Sterile flat shovel
If you have a concrete cistern (especially if it's a concrete block cistern), you may want to also have concrete patch/Thoroseal/Hydraulic Cement on hand to make necessary repairs to the wall of the tank.
Here is the video. Hope it helps!