Here's a few ways I use, not always all in one go but usually a mix. Depends on size of sculpt, level of detail etc.
1) Chill it in the fridge/freezer and buff it with tools and superfine sanding pads.
2) Use freeze spray to chill down and harden specific areas (canned air turned upside down when sprayed, watch your fingers as its VERY cold), then polish with cloths, fingers etc. When the frost melts after a few seconds you're left with a light covering a water which acts as a lubricant.
3) Vaseline or lighter fluid, use this only right at the end and use fingers, soft brushes and soft material to buff up. Lighter fluid cuts more so test on a spare bit of clay to see which gets you the best results.
4) Alcohol torch where you can puff flame to heat small areas and fire polish. You can get these from dental suppliers. Here's the kind of thing
5) Use decreasing sizes of serrated tooth loop tools, start with coarse ones and work through to the ones that have a smaller but more numerous tooth, then to ribbon tools and on to smoothing as above. This method smooths the bumps in the surface initially and then refines it ready for final finish. You can make these tools yourself by cutting into loop tools to give serration and using guitar wire fitted into handles for the finer detailing.