My first post to Permies! Had to weigh in on this one since I am currently heavily involved with setting up the first Seed Library in my community. We open March 2. Just want to share some incredible resources for seed saving. First, a book called "The Seed Garden - The Art and Practice of Seed Saving" Very thorough, clear instructions, great pictures. This was put together by Seed Savers Exchange in Iowa. They also have a wealth of information available on their website for learning how to save seeds. They give separation distances, how to hand pollinate, how to ferment tomato and cucumber seeds, etc.
My other big tip (most people probably know this, but for those that don't)... Make sure the plants you are saving seed from are open pollinated varieties. If you save seeds from a hybrid plant, they will not grow true (turn out like the original plant). Saving seeds from a tomato, squash, or melon you buy at the grocery store could result in a big disappointment after growing them out the next year if that original fruit was from a hybrid. Start with open pollinated seeds from a reputable source and go from there so you get the best results possible.
Spread seed saving. It's one of those skills that has been lost by most gardeners, but is more important than ever!