You may want to start with these examples, then try your own. You can also get some ideas by looking at the fascinating word clouds for each candidate.
Click on a word to see the charts:
The word "yes" is not common in formal speeches. The large spike visible in Obama's chart is from the 1/08/08 speech given on the night of the New Hampshire primaries, which included the refrain "Yes We Can".
Obama mentions the word "wife" more often than McCain.
Click on a word pair to see the charts:
obama, mccain - Obama referred to Mccain heavily throuhgout the primary season. Mccain began referring to Obama heavily only once it became clearer that Obama would be the presumptive nominee.
obama, clinton - As the primary season progressed, McCain referred to Obama more than to Clinton. Obama referred to Clinton more heavily in the primary season.
hope, change - Although he used to mention "hope" more than "change", Obama now mentions "change" more than "hope".
good, evil - Both candidates mention good more than evil.