1. Always use semicolons. Implicit semicolons (in JS) are a horrible idea, especially with some of the interesting syntax floating around in common usage. They are also generally required by any JS minifier.
2. [Avoid eval()][eval]. This causes all sorts of problems, and is a very quick way to slow down your application. Most uses are actually abuses. Every time you think you need to use `eval()`, double and triple check for another way; there is almost always a cleaner, easier way unless you are actually trying to execute an entire string worth of JavaScript.
3. `(function() {/* stuff */})()` is a good way to encase a set of code and create a local scope for it. Using objects is another, often better, way; objects are more analogous to namespaces in other languages when used in this way. Just watch out for `this`. Unlike most other languages, `this` doesn't always reference what you might intuitively think it does. Also remember that unless otherwise specified all JS variables, functions and other objects are all global, even across multiple `.js` files.
4. Always use semicolons. Seriously. Get an IDE that warns you when you forget them.
5. I can't think of a fifth right now.

  [eval]: http://blogs.msdn.com/b/ericlippert/archive/2003/11/01/53329.aspx