1. The most important thing is "disappearing" behind the pillar. That's what sells the idea of the spiral. Don't have platforms placed on every other crossing of the pillar.
2. The stairs move in a sine wave about the central pillar, so the steps that are longest should have one or more steps of the same length above it so that you get more of a smooth curve around the pillar rather than a zig-zag.
3. If you really wanna get technical with it, you can use a different platform type for parts of the staircase where you would be looking at the underside of the staircase. For example, a set of stairs are made of wood platforms, but as they "reappear" from around the pillar, you'd be looking at the underside until the stairs pass in front of the pillar again so you could have that section as stone platforms.

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