Trim letters: In just about any font, u and n can substitute for each other. M and W are natural substitutions, as are b, d, p, and q. With a little snip, it's easy to make the letter c from an a, b, d, e, g, o, p, or q. The letter t can become an f. In this layout, the u is an upside-down n and the lowercase y is actually an uppercase X.
Stamp the missing letters: If I can't spell all the words of a multi-word title, I stamp the words with the missing letters.
Mix fonts or colors: On a whimsical page, I'll mix fonts and/or colors in the title words. Here I used a variety of colors in similar fonts to echo the colors of the party. (Also, notice that the two p's aren't quite the same. The 2nd is an inverted d.)
Trace the letters: If I'm short a letter because my title uses that letter more than once, I trace the chipboard. In this example, I had an e, g, and s, but not a second g. I traced the letters for a uniform title and can now use the chipboard letters on a future project.
Use the negative: I never throw out the negatives from an alphabet set. They can be used as stencils or to fill in for a missing letter. They work especially well as the first letter in a title.
Use a button, flower or themed shape for the missing letter: Here I had all the letters except an o. Because it hit 100° during the trip, a sun was a natural stand-in for the missing letter.
What other tricks do you use to deal with a missing letter? Please share your tips!