Crayons, especially wax crayons, can be applied to a bisqued pot as a wax resist. They can allow you greater freedom in attaining thinner, crisper lines and are especially valuable for linear decorations. For these lines to remain after firing, however, you do need to be aware of how runny your glaze will be.
Warming the pot first will make the crayon draw much smoother lines on the pottery. You can easily accomplish that by blowing hot air on the bisqueware using a hairdryer.
For this pot, I fist decided upon the decorative effect I would be looking for. I then glazed the interior, pouring glaze in and then back out.
As you can see, I decided on a slanted-line design moving around the shoulder of the pot. I deliberately used a brightly colored crayon so I would be better able to see the resist once applied.