I have noticed that in many, many tutorials people decrease the size of their banner by 2 pixels in order to fit a 1 px border in. In this tutorial I shall show you how to add a border without having to decrease the size of your banner.
Once you have finished making your banner/piece, follow these steps:
1. Make a new transparent layer above all your other layers (Feel free to flatten/Merge the all your layers beforehand)
2. Go to Select > All
3. Choose the colour border you want using the colour palette.
4. Go to Edit > Stroke Selection and choose the thickness of your border and if you want a pattern, what sort of pattern you would like.
6. Click Okay and hey presto, a border around your banner, without having to decrease it's size.
Hope you found this useful. ^__^