Homeschoolers are in a position to maximize incidental learning. Incidental learning takes place outside of formal lessons or study time. Incidental learning happens when the baby discovers that some of his bathtub toys sink and others float. It occurs when a toddler realizes a puzzle piece only fits one way. It happens when a preschooler suddenly realizes he knows the alphabet, thanks to hearing the ABC song over and over. It can even happen during the course of your school day when Little Brother overhears Big Brother's online homeschool lessons.
Incidental learning tends to be retained more effectively than material presented formally. I compare it to a volunteer plant near your garden. No one intends to plant a seed in that particular spot, but a plant grows there anyway. It does so because conditions are perfect for it to thrive. Incidental learning occurs when a student is primed and ready to learn that particular concept at that particular time. Of course, the seed must be present and water must be provided, even if it happens casually instead of on a strict planting and irrigation schedule.
The homeschool lifestyle itself tends to maximize incidental learning. An obvious example is when younger children take an interest in their older siblings' lessons. They might enjoy watching science experiments, listen to computer classes or courses on DVD, or overhear brothers and sisters discussing what they've learned with Mom and Dad. Because homeschool parents know exactly what their children are studying, they are able to provide practice during the course of everyday activities. When baking cookies with a child who is learning fractions, it's only natural to point out the relationship between your recipe's half cup of sugar and that morning's math lesson.
Parents can do many things to encourage a seed of knowledge to take root. A big map, posted in a prominent place, is a valuable geography aid. Be subtle in demonstrating its use. When discussing vacation plans, point to your location and the intended destination in a matter-of-fact manner. If a news story catches someone's interest, wander over to the map and find the appropriate region, as if you are doing it for your own information. Your children will come to see maps as everyday life tools, instead of associating them with "school".
I like to display small posters, without comment. Within a month, everyone in the family knows at least some of the material on that poster! Those laminated placemats that are printed with U.S. Presidents, states and capitals, and other educational concepts make terrific posters. Put one in a busy location. (Eye level in front of the main "seat" in the bathroom is an excellent place to display these.) Don't overdo it. One poster at a time is plenty.
If you want your child to memorize something, don't assign it as a lesson. Read it to your family every, single day for a month. Chances are, it will be memorized effortlessly. The key is daily exposure to the material, whether it's a poem, a Bible verse or the Preamble to the United States Constitution (which I recently "tricked" three of my kids into learning).
My preschooler's room is decorated with posters intended for the classroom. This type of poster is much less expensive than posters designed for bedrooms. (Mine came from the Dollar Tree.) They are a great way to attractively present concepts like color, shapes, numbers, and letters. Of course, you don't want to deny your child his favorite Thomas the Tank Engine poster, but try sneaking in a few alphabet pictures, too.
This is a small sample of things that have worked well for my family. I'm sure you can think of examples of incidental learning from your own experience. Please consider sharing your examples in your comments, for others' benefit and enjoyment!