I sign my pots two ways: I write my name using a blown-out pen or a pencil--whatever is handy. My sig is pretty consistent, so it works well on a piece that will have a blazed bottom.
Other times I use the metal die for an old rubber stamp I had made years ago. The old stamp is long gone, but the die comes in handy because it's done in reverse and looks great on clay. It depends on the pot, mostly. The stamp is excellent on a flat surface such as a tile back or the bottom of a slab-built piece.
Care has to be taken when using a stamp to keep from creating stress in the clay on the reverse, but the stamp should be pressed against clay that has a good reverse support--either your hand or another flat surface.
I do also have a silicone stamp that comes in handy at times. You can go to your local rubber stamp store and order one. All you need is a well-drawn black and white image they can use to make the pattern.
You can make your own stamp out of fine-grained clay and a woodcut/linoleum tool called a veiner. The veiner can be found at an art or woodcraft supply store. The blade is shaped like a sharp V. Let the clay dry to bone hard, pencil on your design and carefully cut it into the surface. (Be sure to keep in mind the clay will shrink, so you need to make very strong lines.) Bisque the stamp and, if you want, fire it to vitrification. The incised design will be raised letters/symbols when you stamp your clay.
I did this to make a label for a company and it turned out well. (Don't have any samples, though.)