It's the same tip we featured back in 2008, but things have moved a little. In System Preferences, load the Keyboard pane and choose the Keyboard Shortcuts tab. If you have a lot of Services and/or Applications installed, there's sometimes a delay (read: SPOD) before it lets you get down to it. It's ok, I'll wait.
Ok, next we open up the Application Shortcuts menu on the left side of the window. Once that's selected, pressing the plus (+) button under the right panel will let us define a new shortcut. Press it, and then choose Safari from the Application drop-down. Here's the only mildly tricky part: the Menu Title field has to exactly match the text of the menu item you want to add a shortcut for. In the case of toolbar extensions, each one added is going to have a Hide/Show menu item under the View menu. Take, for example, the Regexp Search extension. It adds a regex-based, "Find in page" search bar to Safari, accompanied by a "Hide Regexp Toolbar Bar" menu item. That's what I typed in for my first shortcut item, and then assigned Command-Option-R to it. Next, I added a second shortcut, using the same hotkey, for the menu item "Show Regexp Toolbar Bar." The two menu items alternate, so using the same hotkey effectively creates a toggle.
Quick tips: If I'm adding more than one shortcut, I like to grab quick screenshots of the app's menus in Skitch (or just with Command-Shift-3 or 4) and keep it visible so I don't have to keep switching to verify capitalization, ellipses, et cetera. Speaking of, the ellipses (...) is not three periods, it's a special character you can type by pressing Option-; (semicolon). That's handy for a lot of menu items you might want to add shortcuts to.
I know there are only so many keys, and so much space in muscle memory for remembering seldom-used key combinations, but fortunately there aren't an overwhelming number of really useful toolbars to contend with, yet. I hope this makes the ones you do find useful even handier.