Make your strikethrough a simple keystroke:
1. Create a word with a strikethrough using Format->Font->Show Fonts to bring up the font inspector as Marko said. Highlight a word and apply the desired strikethrough.
2. Next, using the highlighted word you used the strikethough on..... Select Format->Font->Copy Style (option-command-C)
3. Apply your strikethrough as often and wherever you want to using the simple keystroke (option-command-V)
Note: This doesn't preclude you from copying and pasting text too and from the clipboard at will.
The Style will remain in the clipboard until you. Any text you copy paste can be changed without interfering with the style.
sidenote: not sure if the style will stay if you close notes, but I always have mine open so strikethrough is always available by keystroke. No more surfing through menus every time.
I can even apply strikethough to other text such as in stickies after copying the style!!! Haven't tested it with all apps however.
Mountain Lion 10.8.5