It is easy to fall in love with soft clay. It responds quickly to touch, the mark of one finger leaves a clear impression, a simple pinch will raise a wall, a light pull changes the curvature of a form. It is for that very reason that your early hand-built forms tend to collapse. A pinch pot may spread wider than you planned, a small sculpture may slump at the base, or a rim may lean over if you are touching the clay too much.
The first thing you need to do is think of soft clay as a material to coax into shape rather than a lump to be squeezed. Pressing too hard causes the pressure not only where your fingertip lands. It is felt through the walls, the base, and the edges nearby, and so one strong pressure can crush a curve or flatten one side of a wall. Light touches that repeat, for hand-building, work better than a single strong correction. Pinch, turn the clay, check the wall, and then pinch only that section.
One way to understand this is to do a small experiment using the same amount of clay for both forms. Form the first quickly and hard, and as fast as you can. Shape the second more slowly, with smaller movements, turning the clay every few touches. Notice that on the second piece, the rim and wall are not as likely to be flattened and it should be easier to reshape the walls since you haven’t pushed the clay too hard or too far in the beginning.
Adding more water won’t always help if the surface is shiny, too sticky or too slippery. It will not make smoothing or shaping easier because it will also weaken the walls and you will end up smoothing with the sticky surface, unable to keep the clean shape you want. Be careful when using a sponge. It’s a helpful tool but it shouldn’t replace your hands every time a mark or dent appears, and sometimes the best strategy is to stop working with the clay until it begins to harden a little before adding feet, handles, texture, or some other detail.
Support also helps. When you are shaping a small pinch pot, keep your outside hand outside the form to gently hold it in place as your inner hand works on the inside, no need to crush the wall, but it will need a stable base to work against. Support the base when working on a sculpture. If the bottom of a figure is too soft and heavy, and you keep adding details on top, it will slump.
It is also helpful to see a piece from different angles. A piece can look okay at first glance, but once you see it in profile you can often see a wall that has slumped, or a thick base, or a rim that has become lopsided. Check from different angles, turning slowly on its edge, from above and then look at the base again. The more often you check your clay, the better you are able to see small shifts or lumps, which you can gently correct with a squeeze, a smoothing, or by putting the clay down for a while.
Your first piece doesn’t have to be perfect to be satisfying. It should give you something concrete you learned from working it, like the effect of pressure, or what clay wants to do over time. If the form has flattened, find the spot where you pressed too hard, if the rim has wavered, did you make one side of the wall thinner, and if the surface is too sticky, how much did you add? The next piece is likely to work better because you now have a clearer sense when to shape, how best to support, and when to leave your clay for a bit.