One real nice feature available on Geocaching.com is the ability to create a pocket query of caches along a specified route. You can use Google Earth's Search/Directions feature to set up your route, and then upload that route to geocaching.com. The entire process is described on the "Caches along a Route" section on the "My Account" page.
It's fairly straight forward, but let's run through a quick example. Let's say you plan on driving from San Francisco to Wheatland next weekend for GeoWoodstock VI. First, enter the information in the search box in Google Earth:
Then, right-click on the top layer of your search results (here, it's "San Francisco, CA to Wheatland, CA") and save the file in KML format to your hard drive. Now go back to the Caches Along a Route page in geocaching.com and click on the "Upload GPX/KML" tab. Click on "Browse" to select the KML file you just saved, and then click "Upload":
On the next screen that appears, select "Preview" to make sure everything looks O.K. Then, check the box next to the search title (you can change the name if you like), and click the "Save Selected" button. Now select the "Your Created Routes" tab, and you should see your new creation at the bottom of the list:
If you want to inspect the results one more time before creating a pocket query, click on the link to the left and you'll see a page with the route information and a map:
If all looks good, then go ahead and click on "Create Pocket Query", then select the buffer along the route to include in your cache search (for this route, a 0.5 mile buffer yielded over 500 caches).
And that's all there is to it. Now you've a got a pocket query containing all the cache hides between you and GeoWoodstock VI, which should make for a nice trip to Wheatland, CA.