Proper equipment and good work conditions are half the battle. We will make some general comments that we deem important for a good sanding.
During the sanding operation
Hand or Machine sanding
Preparation. The better prepared you are, the fewer surprises you will get underway.
Buy quality paper. To test the quality:
Flex / fold the paper. Will the backing material easily become visible or if the abrasive side delaminates or flakes off, the quality is often poor.
Abrasive grains should be firmly in the paper - the grains should not fall off, but be worn down during sanding.
The paper should be stable but yet flexible, so it does not break.
Good light. An adjustable work lamp that can light directly onto the surface at different angles will be a good "helper".
Protection. Be sure to have a dust mask and preferable goggles when sanding, especially during machine sanding.
Instructions. If you will use a machine? Read carefully the instructions from the machine supplier before use.
During the sanding operation
Be patient and work structured.
Change the paper often. Remember to change paper when it begins to lose sharpness. It makes the job more efficiently and far more satisfying.
Remove dust in the way. This reduces the risk of clogging the paper and you get a better removal rate and the paper will last longer.
Use your fingertips. There are a lot things an eye cannot see but that the sensitivity in the fingertips can detect. And do not be fooled: these fingertip irregularities will appear visible after you apply the new paint / varnish!
Grit size Change. Do not go up more than 1 to 2 grit sizes a time. Each grit size is intended to remove the marks from grit size below. Skipping too many steps, you will just need to work longer. Do not change the grit size until the marks from the previous grit are removed. Remove dust between each grit change.
Do not start to rough. Not sure where to start, try gently forward. With the exception of rough surfaces, 80 or 100 a good starting point.
Wood ends. Remember that the wood ends are harder than the surface of a piece of wood. Therefore, use a degree finer grit here than the rest. This is to avoid unnecessary scratches and that the wood should not be darker than the rest of the surface after the applied finish.
Sharp edges. You may want to round off the sharp corners / edges. This reduces the risk of peeling and the paint will attach itself better. You also reduce the risk of sanding away the applied paint / varnish on these vulnerable spots.
When to stop? This depends on what finish you want. A thicker surface finish such as oil paint will cover more than a thinner finish. Important note: Surface finish that penetrated down in the wood requires that the entire surface is sanded with same grit sizes, otherwise it will not be absorbed by the wood evenly and the color may be uneven.
Hand sanding or machine sanding?
The selection of sanding method will largely depend on what will be sanded and the size of the job to be done. Large, heavy sanding jobs like sanding of floors, stairs or other large surfaces are best and most effectively done with a machine. Some jobs you will simply not be able to do by hand. It is easy to get lost in the different sanding methods that can be used. Consider the need, understand the process and do not become distracted by things that do not really have any significance to your particular job!