Summarize the following:
For most cacti species, you will need to repot the plant once its roots begin to show through the drainage holes of your pot or once the crown of the cactus reaches the edge of the pot.  This will usually occur every two to four years.  Repot your cactus during the dry season, usually in late winter or early spring. Roots may break during the process, and moisture can cause those broken roots to rot. Slip on thick leather gloves. The material must be thick so that it can help protect you against the prickly spines of the plant. Gloves alone may not be enough to protect your skin, but even if you take other protective measures, you should still include thick gloves among them. Run a dull knife around the inside perimeter of the pot, using a sawing motion to help break up the soil there. Continue as needed until the soil starts coming up as a solid mass.  If you have a plastic pot, you could also try squeezing the sides of the pot to loosen the soil there. Tap along the sides of the pot with a dull knife to help loosen the soil, as well. The soil surrounding the root mass should be completely loosened before you remove the cactus. Otherwise, you could end up damaging the plant. Stack a few sheets of newspaper together and fold them in thirds to create a thick, sturdy strip. Wrap this strip around your cactus. Carefully grab this strip, pressing it against the cactus as you work, and lift both the strip and cactus up and out of the pot. Alternatively, you could skip the newspaper and use an old pair of barbecue tongs to lift the cactus out. The idea is simply to keep your skin as far away from the spines of the cactus as possible.

Summary:
Know when to repot. Put on gloves. Loosen the soil. Lift the cactus out with newspaper.