Tips and tricks
To say I learned a lot building the treehouse would be an understatement. It's basically a small wood-frame house that just happens to sit on a tree stump. Here's a few of the more important points:
Learn how wood frame houses are built. It's really much easier than I thought it would be, but it helped a lot to read about it first. I liked HOMETIME How-To: Framing and House Framing: Platform & Balloon Frame Construction.
Design. I spent two months just drawing blueprints and 3-D views, both to figure out what I wanted and how it all fit together. If you've never built something this complex before, there are a lot of things that simply aren't obvious. For example, three different pieces of wood come together at the corners and they can't all occupy the same space. How does that work? And how do you get the deck to slope nicely so that rain doesn't puddle on it? How do you attach the railing posts so they can hold the weight of your neighbor's five-year-old? It goes on and on. There are lots of resources on the Web, such as this article on building a garden shed, this article on building a playhouse (PDF), and this article on building a treehouse.
Plan your purchases. A benefit of accurate design is that you can plan how much lumber to buy and how to cut it. If you just start buying and hacking, you'll spend a fortune and end up with a lot of unuseable bits and pieces. Spend the time before you buy to draw how each stud and piece of siding will be cut from the wood you buy.
Use good tools. Don't use your treehouse as an excuse to buy every toy you ever wanted from the hardware store -- you'll spend enough money just buying the wood. But you also shouldn't skimp when you really need something. I used a hand saw and chisel for days to shape the stump before I finally went and bought an electric chain saw, which finished the job in a few hours. Other invaluable tools were a mitre box for cutting studs straight and a circular saw for the siding.
Learn to toenail. You can't build a frame without knowing how to toenail. A friend of mine let me in on a nice trick for working with dry wood, which splits when you try to nail it. Put the end of the nail on concrete and hammer on the point to flatten it. The dull tip will bash through the wood rather than spreading it apart, which a sharp tip will do. (Sharp tips are fine for the wet wood you've just bought at the lumberyard.)
Cuts have width. Yes, that saw of yours is not infinitely thin. If you cut directly down the middle of your line, or to the wrong side of it, you'll end up with pieces that are an eighth of an inch to short. This might not sound like much until it starts raining. Align your saw so that the cut is along the outside edge of the line. And don't forget to include the width of the cuts when making your plans -- you really can't get four 2' studs from an 8' 2x4.
Build a good foundation. I spent a lot of time getting the foundation level within about 1/8". This paid off when I built the rest of the house, as I didn't have to worry that the walls would tip over.
Get a friend to help with the siding. Siding, which looks easy, turned out to be the hardest part of the project. If you don't get the first piece on straight, the rest will get progressively worse. If I'd been building a normal-sized house, there would have been a six-inch gap at the end. Learn to do it right.
Spend a few extra bucks on the roof. It rains fifty to sixty inches a year where we live, so you can't spend enough on a good roof. I could have done a better job -- glueing instead of nailing -- but at least I remembered the roofing paper, drip edge, and peak shingles. HammerZone.com tells you all you need to know.