Constrain your hypertufa in a mold or form for the best results. But what do you use for a form?
Look around you; I'll bet there are some molds or forms right in front of you. They can be free or cheap, plastic bowls from the thrift store or a garage sale.
I've made great garden planters from all kinds of found objects, from baskets to styrofoam.
Sometimes it's best to use a form release such as vaseline, Pam or a liner of thin poly (or not so thin; check out the page on textures).
You'll look at things with new eyes whenever you're shopping or browsing a garage sale; anything with some flex, like a plastic container that is bigger at the top opening than the bottom will work.
Baskets woven from wicker or twigs can have one last hurrah by being used for a form.
Line your basket with very thin plastic film, like that which comes from the dry cleaners, or used for drop cloths when painting. The basket is sacrificed in most cases, as you can't get the completed pot out of it without cutting, or burning it off.
Buckets, round or square, plastic ice cream containers, even planters make great molds. Cut the top off if required. These can be reused as often as you like - again, line with poly. In this type of form, you can line them with thicker poly, which creates wrinkles and more texture. For ultimate texture, line with bubble wrap.
You don't have to fill the molds up completely; I've made some really nice shallow planters for succulents by only making the hypertufa pot a few inches tall in a large square bucket.
Sag pots can be draped over a pile of sand by pressing the hypertufa mix over it to form a bowl upside down, or make it the other way by digging out a depression and pressing the mix into the sides. In both cases, you have to leave these in place for a long time until the curing has taken place.