USING TAPE RESISTS
You can do some interesting things by using tape as a resist. You can make straight or wavy lines, and geometric shapes. Here are some ideas.
ABOUT THE TAPE
Masking tape is the most basic option, and while it has some disadvantages (it can absorb, it can tear while you’re pulling it up, and it doesn't stretch), it can be a good place to start.
Blue painters' tape is probably a little better. Electrical tape works well and stretches somewhat. Another option is Chartpak which is a graphic artists' tape and comes in various widths. Drafting tape is another option, as is auto pinstriping.
Make sure the tape is stuck down well by burnishing the edges with a fingernail, edge of a spoon, or wood tool.
Remember that the wider the tape, the harder it will be to get it to stretch evenly over a curved surface. But don’t stretch too much or the tape will try to pull loose.
ABOUT THE TECHNIQUE
If you can apply glaze with a sprayer, you will probably get the best results because you won’t get oozing under the edges of the tape. But other techniques can be used as well.
It usually helps to peel the tape off before the glaze is completely dry. You can apply tape to bisque, then apply your underglaze or glaze. This is especially pleasing with Raku where the masked lines will turn black.
For a more subtle look, you can pull the tape off and then cover the entire surface with a second glaze. You will get a color variation where the resist was, creating a very interesting surface. A variation on this would be to use underglaze as the base layer, or a chemical-like rutile which causes glazes to break.
This subtle approach is especially good for people who are not detail-oriented enough to do a fully geometric pattern, as it doesn't need to be precise to look good. I, for example, am not one for extreme contrasts. But I find two different tones of one color applied in this way to be very pleasing. Think about two different shades of red. Or even two neighboring tones such as green and turquoise, or blue and turquoise. Remember that glazes can run when layered so test because it can end up looking great or really bad.
If you do like precision geometric patterns, or just want to do them more precisely than you have in the past, check out the MKM Decorating Disks. They really do help section up the piece and draw the spacing lines on accurately.
You can also tear masking tape lengthwise to get a torn surface. For larger areas, try tearing paper towels or newspaper and applying when wet.
There are many other substances that can be used for masking, such as wax. But tape has the advantage of being easily re-positioned until you are happy with the pieces. Give it a try!
Even if you don’t particularly like geometrics, you can create some fluid, organic designs. Just a small patterned band at the top or bottom of your piece will bring it to life.