There are so many keyboard shortcuts, it gets daunting, I know. But there are a few that are a must for speeding up your workflow and eliminating errors while editing.
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The first to know is the "a" key. This is the "Select Tool" which is your default arrow tool. You should be in the habit of pressing the "a" key after you finish using any other tool. The reason is that if you use, say, the blade tool, make a few cuts, and then try to immediately select the part to cut out, you will only be making another cut. You can't turn a tool off, you can only select another tool. Since the Select Tool is fairly non-destructive (the Hand Tool being the most non-destructive) you are safe from making errors that have to be undone.
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The second is the "cmd-z" keystroke. If you make an error with your tools (or anything else for that matter) you don't have to go to Edit/Undo, you can just press "cmd-z" (CMd is the abbreviatioin for the Command key, which is the one with the "Place of Interest" symbol on it. Looks like a cloverleaf...) Keep in mind that you have limited UNDOs, so don't go to far from an error.
p.s. If you want to experiment with any filters or text in a sequence, create another sequence, copy your timeline from sequence one into sequence two, and do the experiments there. If you like what you did, you can always copy and paste back into the original, but if you don't like it, you don't have to worry about the undo limits, you can just delete the new sequence...
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The third is the "n" key, which is used for toggling the snapping feature of FCP. Snapping is imperative when you are placing clips within the timeline, but a real pain in the butt when you are trying to navigate (scrub) through the timeline. Especially if you want to get close to an edit point to see those last few frames, right? So you have to either zoom way in, or keep clicking the snapping button on the right of the timeline. The "n" key will toggle this for you.
p.s. A new feature of FCP 6: if you start dragging in your timeline, and then press the "n" key, it will toggle snapping on or off (depending on its previous state) only for the duration of that operation. When you let go of the mouse, snapping will return to its previous state. This is very handy, as it reduces the amount of times you need to press "n" by half. And, you cannot use this feature without the keystroke "n", because you must keep your mouse in the timeline.
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These are the most commonly used functions. So keep your left hand (if you're a righty) on the keyboard and get used to hitting these keys. It may take a few hours of using FCP to get used to not using the mouse, but trust me, break the habit. Force yourself, even if it takes longer to edit a particular piece, force yourself to use the keystrokes instead of the mouse. Once it becomes natural, you will be surprised at how much time it will save you.
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Some other good strokes to remember:
"j" - plays a clip in reverse at variable speeds (keystroke is cumulative)
"L" - plays a clip forward at variable speeds (keystroke is cumulative)
"k" - stops a clip from playing (does not start playback)
(these three keys are a great "Home Row" for FCP. No more clicking play buttons in the
viewer/canvas.)
"b" - switches between the Razor Blade tool, and the Razor Blade All tool
(press "b" twice quickly while in the timeline window and you will get the double blade icon.
This will allow you to cut through the entire timeline, instead of just one clip.)
"shift-L" - toggles Linked Selection. This will allow you to choose just audio or just video in the
timeline without losing the A/V connection. Good for L-cuts.
"shift-z" - fit to window _ works in the timeline, viewer, and canvas - fits all info into forward window

Learn them as you need them. Don't try to memorize them all at once.

Enjoy!