First, if you are like me and your vehicle well maintained then you only need to do this about once a year or less. What I look for is to observe your paint surface when it is clean and in direct sunlight and seeing it reflect off your paint which will easily reveal swirl marks. No matter how careful you are when washing, eventually you will put minor swirl marks in the paint. To combat this and other types of scratches there are a few things you can do.
The first thing to do is periodic maintenance like removing any hard water spots or localized swirl marks. This happens to me from time to time when its been raining. Does this after you have washed and dried the car then get yourself a good quality swirl removing polish and a good quality polish applicator and apply a small amount to the area you want to polish. Do this in a shaded area and you will get better results. Gently, in a circular motion, rub on the polish until the spots are gone then buff with a soft microfiber towel. You should see a nice clean shiny area of paint. This method is for spot applications, not the whole vehicle as this method would take too long. After you have finished with all of the spot work, make sure to wax all of the areas that were polished. If you are using a polish that is not so fine then make sure to follow up with a fine finishing polish to complete the job.
If you are going to do the whole surface of your vehicle such as yearly maintenance, then you will want to use a power polisher which can cover a much larger surface than hand polishing. There are two types of polishers you can use for this. The first is the circular polisher and the other is an orbital polisher. For this discussion I will be talking about the orbital as it tends to be used more by the weekend car buff and the circular is more geared for the professional car detailer.
One of the most popular orbital polishers out there is the Porter Cable 7424. It has a variable speed control so you can use a low speed for polishing and a higher speed for buffing. It also has an orbital action that will not allow you to damage your painted surface.
There are different pads for different types of polishes or waxes that you would use. I try to use the Adams family of polishers as they are very good quality for a good price. Start with a number 3 polish and finish with a number 1. I would also recommend using a hand buffing to wipe away the polished surface with a microfiber towel. For scratches that are deeper than swirls, go to a more abrasive polish and see if that does the trick, always stepping down in grit to finish off.
One of the things that you will notice about the orbital style polisher is that if you put to much pressure on it, it will stop rotating. When using, maintain a light pressure on the pad so it does rotate. Sometimes it is hard to see if it is rotating so I took a magic marker and drew a line on the top of the plate to show rotation.
Use a pad that is designed to be used with the grit polish you are using. Work an area about 2 square feet or less. Apply some polish to the pad at four or five spots on the pad and put on the paint. Make sure that you hang the power cord over your shoulder to keep it away from your paint. Turn on the polisher at a low to medium speed and slowly apply the polish in an even circular motion. When working in the polish you will notice that it begins to dry and has a change in state. At this point stop and turn off the polisher with it still on the paint and remove. Buff the polished area and examine to see if you need to do more. When finished, if you were using a medium grit polish, repeat using a fine polish.
Once the whole surface has been done, wax the surface to complete the job. You can use the same orbital polisher to apply the wax if it is a machine wax. Once you are done with the pads, remove and hand wash to remove any polishes or wax so they can be used again.
At this point you will probably have spent about an hour and a half to buff your vehicle but the end result will have made so worth it.