This gimp video tutorial touches base on a very powerful tool that if used properly can really improve on your photo editing. Layer modes allow you to dynamically change how your image appears by modifying how the layers underneath look.
In my Gimp Tutorials, I frequently use Wacom Tablets. Here is an awesome wacom tablet I recommend using.
A Wacom tablet will enhance your GIMP photo editing experience by offering a pressure-sensitive touch that translates to varying transparency when making brush strokes in GIMP.
Gimp Reference Manual
I highly recommend getting this Gimp book to use as a reference manual in conjunction with my tutorials. This book will explain how the tools I use work. My tutorials will give you practical applications for the tools shown in this book.