Sanding: Always sand in the direction of the wood grain.

Let's start off with some extra instructions for the unfortunate souls who had a poly/epoxy/catalyzed finish, and were left with islands of old un-stripped finish on the body. If you have a finish that was completely removed by the stripper, you should still use these methods, you just don't have as much work to do:
The best way to approach removing unevenly distributed areas of finish that the stripper was unable to loosen is to use a very sharp chisel. I say this because it is the method I use, but I do it because I know how to control the chisel and can use this dangerous (to your guitar) tool without removing layers of wood along with the finish. If you feel confident with a chisel, then that is what I would recommend.
If, however, you do not have the experience to do this without gouging chunks out of your guitar body, you should use a large sanding block that will hold an entire 8-1/2"x11" sheet of sandpaper. This can be made from anything that is flat and not flexible. A piece of melamine shelving from Home Despot or Lowes will be perfect.
Depending on how much material you are trying to remove, start with either 180 or 220 paper, and progress through the grit until you end with 320.
There are other options, but the whole idea is that you should not use a smaller sanding block in this situation due to the fact that the smaller sanding block is going to also sand the bare wood areas around the islands of old finish you are trying to remove. This will leave you with a less than level surface that won't be apparent until you put finish on it. The "whole sheet of sandpaper" sanding block will ride on the high spots created by the left-over finish, and will sand them down to the wood surface pretty evenly.
If you can't find an appropriate sanding block, you can use double-sided carpet tape to hold two sheets of sandpaper to a truly flat surface, and move the body back and forth over this. Remember to move it in the direction of the grain, and keep it moving straight.
Either method is going to work great on the front and back, and the smaller sanding block will let you work on the sides of the body and the waist contour on a Strat, but...you are going to have the areas on the inside of the cutaways to deal with. This is where the 1" dowel is used, and it may take some time to remove stubborn areas of left-over finish.
Take your time, use sanding blocks so you don't distort the surfaces, and you will do a great job.

Cautions:
Ash bodies take an extra measure of caution, because Ash has areas that are of different densities. It is easy to sand edges and corners on an Ash body and leave a rippled looking surface because of this grain quality. Use a block, not your fingers.
Be extra careful around the heel pocket. It is easy to distort this area if you are careless, especially the small radiused edge where the treble cutaway meets the treble side of the neck pocket. Doubly so on a Telecaster, where it is a thin edge.
Use a hard rubber sanding block to get finish off of the radiused edges, and be careful not to oversand.

Final sanding check:
When you think you have removed all of the old finish, dampen some paper toweling with mineral spirits and wipe the body with it. Look at an angle that allows you to see the wood absorbing the mineral spirits. What you want to look for are any shiny areas that are not going dull almost immediately. This will show you where any finish is still on the surface, and you will need work on any of these areas to get all the finish off before continuing. If you don't remove it all, the new finish will shrink or sink unevenly, leaving these areas visible on the surface of the new finish. Do any additional sanding with a block, and don't just concentrate on the specific spot; work the entire area around the spot to avoid sanding a depression that will be seen on the surface of the cured finish.

The next post will cover pore filling for Ash or Mahogany bodies, applying sanding sealer and finish application.

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"There's an arse for every chair"

5 guitars
7 amps
4 dogs