Choosing keywords for your blog, or maybe just a single post on a topic you don't normally cover?
Make sure you consider more than just the monthly volume for the keyword when using Google's Keyword Tool.
Take a look at the competition for the keywords as well.
What you may find is this: high-volume search terms with a LOT of competition and medium-volume search terms with low competition.
The latter may be your better choice.
Here is an example for choosing keywords, just to illustrate the column that shows the competition for a search term:
If you download your searches into a comma-delimited file, you will find that the competition field is a number between 0 and 1. The closer to one, the higher the competition. You can then use this field to help determine which keywords you are going to target in your blogging - by using filters and/or sorting the data until something pops out at you that make sense.