If you want to be in the know about Leopard's new features and how they can improve your work and life, you could wait for a book, sift though OS X tips sites, or have insider secrets leap into your hands the moment you need them.
The first step is to abandon presumptions about what Apple couldn't possibly have built into Leopard and its bundled apps. These presumptions lead to premature workarounds for absent features that actually exist in Apple's code. It's folly, bordering on hubris, to imagine that you're the first person who ever needed to do a given thing. Instead of assuming that a feature is missing because it isn't where you expected to find it, ask your Mac where that feature is.
Whenever you need to do something you haven't tried yet, turn to Help first. Query each application's Help; the Help for Leopard as a whole is linked to the Help menu that's available when Finder is selected. If your query fails, your terminology might be to blame. Go to the Help table of contents and drill down the hierarchy until you narrow in on what you're trying to accomplish, or its nearest identifiable neighbor.
If you lose your way in System Preferences, which is much harder in Leopard since Apple did a major reworking of the most complicated Security, Sharing and Networking preferences, enter a keyword or two (e.g. "firewall") into the search field in the corner of the System Preferences window. Preferences panes matching your query will light up. Most buttons and fields pop up brief, useful descriptions when you hover over them for a few seconds, and you'll often see questionmark buttons on dialog boxes that open context-sensitive Help.
Enter "shortcuts" into any Help window's search field. This will point you to a list of keyboard shortcuts for the app or, using Help while Finder is active, for Leopard. Even if you're not a fan of shortcuts, Apple wires most important and most commonly-used interactive features to the keyboard. Reading the one-line shortcut descriptions is much easier than pulling down every app's menu bar item and trying to figure out what it does.
I spell this out in such detail because Help is grossly underutilized on the Mac, especially among Mac users who are just too cool to click Help. Leopard's Help is concise and easily navigable. Use it when no one's looking if you must, but no matter how good you are, you will always find happy surprises in Help. Covert use of Help has made many a superstar on OS X tips sites.
If you still can't figure out how to make your Mac do what you need it to, open Automator. It drives GUI apps in ways that you'd never have imagined, and if Automator can make an app do what you want, then so can you.