The easiest way is to get a camera that has GPS built-in. Then, every photo you take (outdoors), will have GPS information as part of the meta-data, and they will automatically get placed correctly when uploaded to Flickr or other online sites that support geotagging. However, such cameras haven't yet made it into the mainstream, or even in the prosumer space. There are some attachments for professional cameras like the Nikon D series, but these are overkill for the casual user.

A GPS phone or standalone GPS device can be used for geotagging, with the help of some software. To do this, simply save the track information using TrekBuddy. This GPX file can then be loaded into GeoSetter, a free program available from www.geosetter.de/#Download. The site is in German, but the program is in English. Once downloaded and installed, you need to point Geosetter to the location of the photos on your hard drive, and load the .GPX file into it. It then proceeds to automatically geotag the photos, based on the time stamp in the photo. Basically, it looks at a photo and checks at what time it was taken (this data is automatically added by the camera), then compares that with the .GPX file, and pulls the location coordinates for that moment in time and adds it to the photo. Don't worry, it sounds a lot more complex than it is. In practice, it's a quick, easy process.