The plastic painters mixer that you attach to a drill is hard to beat. Before the mixer I used a 1" piece of pvc to stir.
The important things to remember are:
Use cold water, preferably Ro/DI.
Never ever add "buffer" or any other additive.
Slowly pour the salt into the water, never pour the water into the salt.
Mix as hard as you can while pouring in the salt. You need to dissolve the alkalinity ingredients in the salt as quickly as possible to avoid Calcium carbonate precipitation.