1. Get some ideas from the kids (and adults even) who will use the treehouse. A treehouse should fit a kid like the glass slipper fit Cinderella. But you won't know the shoe size if you don't ask.
2. Give the treehouse some character, an identity, a theme--something that sets it apart from mass-produced, off-the-shelf play structures. A treehouse resembling a boat, for instance, can spark some pirate play and also serve as a mountain top or a race car.
3. Keep the ceiling at kid height. If an adult can stand up inside, the proportions will be off. This isn't just bad from an aesthetic sense. A too-tall interior won't have the cozy feeling of a hideaway, a secret headquarters, a cave.
4. Is it really a treehouse if it's not suspended in a tree? That depends. If even one post has to touch the ground, something is lost. That something can be recovered, though, if the posts somehow reinforce the treehouse's character. For instance, I used 5-inch diameter timber bamboo as a sleeve for 2-inch galvanized steel pipes that supported a treehouse with a Japanese look.
5. Have more than one way to get up or down from the treehouse by choosing two of these: wooden ladder, firepole, steps cut into a log, rope ladder, knotted rope, climbing rocks bolted to a tree or post. This makes the treehouse part of a circuit of travel in the backyard, rather than a cul-de-sac (that's French for dead end).
6. Details, details, details. These can be simple or complex, scavenged from a junk shop or bought online. Try secret chambers in the floor, bells, mailboxes, steering wheels, levers, propellers, doorknobs, working shutters, spigots, flags and pulleys.
7. Finally, the two books I have found most useful are Treehouses You Can Actually Build by David Stiles and Jeanie Stiles and Home Tree Home by Peter Nelson and Gerry Hadden. Both books and much more can be found at Nelson's Web site: www.treehouseworkshop.com. These sources cover ideas, construction, tree choices, safety and regulations. Consult them before starting your own treehouse.
And remember to have fun.