I've just started using a really cool iPhone app to geotag my images. The gps4cam app is available for $3.99 on the iTunes Store, and the desktop app (you'll need both) is available for free on gps4cam.com. When you start a shoot, open the app, click Preferences, and set the gap between captures in Standard mode to 1 minute. The documentation says it takes more battery, but several hours of shooting drained my iPhone battery relatively little, so I'd rather have the greater accuracy.
Next, click the Start button which takes you back to the main screen. Click the green "Start a new trip" button. Put the phone in your pocket and start shooting. When you're done, click Exit. It'll bring up a QR code on your phone. Unless you have a macro lens, set your zoom to a fairly long telephoto, hold the camera at arm's length, and take a picture of the screen. You might have to fiddle with the telephoto a bit... you want the image of the QR code to pretty much fill your viewfinder in the short dimension. I take 3-4 images to make sure I'll have one that is clear.
Once I've copied the entire shoot to my computer, I go to the images of the QR code (which will be at the end of the shoot) and delete all but the clearest one. The QR code contains all the GPS data on your shoot (really long shoots generate more than one code.) The code also contains the current time, so shooting the image synchronizes your camera's clock with your phone's clock.
When you process the images (more on this in a subsequent post) the desktop app figures out the precise moments of all your shots, finds the 1-minute-increment GPS locations just before and just after the shot, and interpolates where you likely were when that shot was taken... and then it geotags the image accordingly. I've tested it and found it to be remarkably accurate, even though I'm a fairly erratic walker as I'm shooting, walking this way and that to get the best shot. Let's put it this way... it's accurate to within 2-3 yards in most cases.
I'd thought about buying a $200 GPS unit to plug into my Nikon D300s camera, but it costs money and takes up space in my already-crowded gear bag. This solution is $2.99 and uses the GPS capabilities of the camera I'm carrying around with me already. Cool, eh?