To prevent your piece from cracking make sure you leave your lip thick at the rim of your pot. Work on this part last. To keep it soft and damp, make sure you keep the rim damp by moistening with a sponge and water. Do not however get the whole pot wet because it will become sticky and slippery to work with.
You cannot build a symmetrical pot without your thumb. So make sure when you are beginning to push your thumb in the middle of the clay make sure it goes straight down into the center of the clay. If you turn the clay as you insert your thumb this will help make the opening more central. Also do not forget to take your time and spin the bowl slower and make smaller pinches
Making pots by pinching is not as easy as it might initially appear. You need patience and perseverance to learn this technique. Squeezing too much clay at one time causes the pot to lose its shape. So allow the clay to set or rest before becoming over worked and floppy. You should build more then one at a time.
Created with an image by geralt - "thumb success successful"