I have had good long term success with the following methods, but proceed at your own risk.

Straightening a warped shaft.

(If your shaft is not warped, scroll on down to the Finishing section.)  Sight the shaft's warp: Put the shaft on the kitchen counter or table and slowly spin it in place, looking for how much runout it has (how much any portion moves up and down).  If you decide it's too warped to use, the scary part begins:

Use lacquer thinner to clean and strip the shaft (keep it away from the ferrule and the joint ring, as some will melt!).

Re-sight the shaft and put a pencil mark on the top of the ferrule when the spun cue is on the rise in the center.

Turn on the kitchen sink's hot tap till it's nice & steamy. Give the shaft a bath (20 to 30 seconds is likely enough).

Quickly dry it off.  Now sit on a chair with your legs apart, the ferrule on one knee (pencil mark up) and the joint ring on the other knee.  Put your hand(s) atop the middle and PUSH.  Check it for warp again and re-mark it if needed.  Repeat as necessary.  When satisfied, stand it up in the corner for a few days.

Finishing your shaft

If your shaft is straight but has some dings, put a few drops of hot water on each ding, then let the shaft set over night.  Burnish the dull spots with a glass (as seen below, but just do the dull spots, not the entire shaft).

If the shaft is really scruffy, turn on the kitchen sink's hot tap till it's nice & steamy, and give the shaft a bath (20 to 30 seconds is likely enough).  Let the shaft set overnight.  Now you will need a spare joint pin.  If you do not have a spare joint pin and the shaft has a metal insert, whittle a softwood dowel just a bit too big to slide in.  Taper it a bit, and twist it in, letting the insert cut threads on the dowel.  (Some shafts use a standard machine thread, allowing the use of a piece of threaded rod or a machine screw with the head cut off.)  Now you need a lathe, or a drill mounted in a vice or held down by a very patient friend.  You can support the ferrule end of the shaft with one hand and work on the shaft with the other.

Run the lathe (or drill) and sand the shaft.  What grit paper you start with will be a judgement call, based upon the condition of the shaft.   Using a rubber sanding block I usually beginning with 400 grit paper, then 600, then 1000 or 0000 steel wool.  Spend some extra time polishing the ferrule with the 1000 or 0000.

Clean the shaft with lacquer thinner (remember to watch out for the ferrule and joint ring).

Put a towel on a table, lay the shaft on the towel, and firmly rub the shaft lengthwise with a drinking glass. (I have one with a base that is smaller than the glass, which gives me two points of contact and speeds up the job - see the photo below.)  Repeat until the shaft no longer has visible stripes along it's length (this takes a while).

Buff the shaft with 1500 grit paper and then a clean towel.   You should now have a shaft that shines like glass yet is very slippery. If you like you can go a step further and use a paste wax such as Karseal Cue wax.  Apply it and let it dry, then buff it off with a paper towel.  Repeat.  Then buff it with a clean cloth.  (Some say that it is best to put the shaft back on the lathe and apply the wax with your hand, as the heat thins the wax and opens the pores of the wood, thereby giving a better seal. I have not tried this approach, but expect that there is merit to it.)