Incredible White Mask
One of my firsts picks when we started doing a pick-of-the-week was Liquid Frisket (aka liquid magic) because it is wonderful. If you mention this post I will give you a discount because I love it and I want you to try it.
Liquid Frisket is a more precise alternative to using masking tape, rubber cement, or clear wax crayons as a resist. It gives you a crisp hard edge and won't rip or alter the surface in any way or leave behind a gunky residue. Bonus points, peeling it off is really satisfying.
I like Incredible White Mask because it goes on white but dries nearly perfectly clear, and won't stain your paper (some liquid frisket can leave behind a yellow stain) and because it dries very quickly which is great if you're as impatient as I am.
The same brand sells a tool called The Incredible Nib that you can use to apply the frisket because it will gunk up and ruin brushes, but I've found that you can use a variety of things from your junk drawer. Toothpicks, old toothbrushes, bamboo kebab skewers, and carved erasers still on the end of a pencil make great and fairly precise tools, but the best thing about liquid frisket is that that it really lends itself to experimentation, so get creative!