Canvas size got you down? Feeling cramped and closed-in? Just follow these steps and you may never trek up to the Image menu and Canvas Size dialog box again.
Step 1: Open the image, and press F to go to Full Screen Mode (so that you have a larger workspace).
Step 2: On the Layers palette, make sure that the layer on which the image lives is NOT named "Background." If it is, simply double click the layer to rename it. This ensures that your new canvas space is transparent, giving you more flexibility with the image (think drop shadows, borders, edge effects, etc.).
Step 3: Press C to select the Crop tool.
Step 4: Draw a random sized selection box.
Step 5: Grab each corner handle of your selection box and drag it out onto the gray area, outside of the image bounds, wherever you need the extra space.
Step 6: Press return and voila! Enjoy your new canvas space :)