Any question answered...

So what is the solution?

The solution to what? Making seat posts under nominal size is the solution to getting a proper fit when the seat tube bore might also be under nominal. There has to be a clearance when assembling, which is then closed by the clamp.

The solution to slipping might be one or more of the following:

1. Reduced load (lay off the pies)
   
2. Increased radial pressure (more hoop stress from a burlier clamp)
   
3. Higher coefficient of friction (friction modifiers like Tacx paste, eliminate slippery anodising as raw Al is stickier than Al₂O₃)
   
4. Double clamps. These have advantages which might not be immediately obvious. The clamping diameter on the post is smaller than the clamping diameter on the outside diameter of the seat tube, so greater radial pressure is developed for a give hoop stress in the clamp. The coefficient of friction between an aluminium clamp and an aluminium seat post is typically substantially higher than that between a steel or titanium seat tube and the same seat post.