I don't know how well this will be received, however since it really isn't that complicated to make flawless borders, i won't add many pictures..
On the image you're working on, simply open a new layer, then get out your magic wand
tool.
Click in the center of this new layer, and then go to Edit> Stroke. From the options there, pick the width of your border, and of course it's colour (hint: using multiple px wide white borders, then playing with the layer options gives you those enhanced colour borders that work nicely with most images) From the three options below colour, choose inside, since your magic wand tool will have selected your entire canvas. After this, ignore the other options, since they are identical to your layer options, and much more of a pain to deal with. Press OK and you have your simple border.
For more complicated borders, you can deselect then reselect your border layer with the magic wand, and create borders within your borders for effects such as rainbows, or varying greyscale borders. If you, however, are looking to change your layer options for a specific set of borders and not others (should you do more than one), instead of making your borders all on the same layer, open up a new layer once more, then go back to the layer with the border you'd like to border your border.... border On this layer, use the magic wand as if to add more borders onto that layer, then switch to your fresh layer-option free layer, and Edit>Stroke once more.
Enjoy :o
