Water the rootbound plant the day before you intend to remove it from the container. A moist rootball is healthier and easier to remove from the pot, because the roots and soil hold together. Dry soil falls apart easily, leaving the roots unprotected.
Try the simplest method before attempting to remove the plant using other, more complicated techniques. To remove the pot, place the palm of your hand flat on the soil, with the base of the stem between your index finger and middle finger. Hold the plant securely while you tip the plant and slide it from the container. Don't pull on the stem. Often, the rootball is so solid that the plant slides out easily.
If the plant doesn't slide easily from the pot, put on a pair of gardener's gloves, and thump the side of the pot with the palm of your hand. Tap the pot firmly, but be careful not to break it. If the pot is plastic, lay the plant carefully on its side, and push the sides of the pot inward to dislodge the rootball.
Loosen the roots with a tableknife, and then slide the plant out of the pot if other techniques don't work. Work the knife carefully around the inside of the pot, moving the knife back and form in a jiggling motion to loosen the roots. Don't worry if you cut a few of the roots. A badly rootbound plant requires trimming before the plant is repotted.
If you've tried every technique and the plant won't budge, you might need to sacrifice either the pot or the plant. If you want to keep the pot and start with a new, smaller plant, use the table knife to remove the plant from the pot. Retain as many roots as necessary, because sometimes a plant might actually survive, in spite of the damaged roots. If the plant is more valuable than the pot, take the pot to a safe place, and wearing gloves, carefully crack the pot. Remove the plant, and repot it in a larger container.