I've always compared mounting photos to framing photos. When you mount a photo for your scrapbook, you really are framing it.
There are quite a few options for mounting photos both in color choices and also in shape. I love using multiple colors. You can use contrasting colors or you can use several shades of the same color.
I also will sometimes use a patterned paper to mount my photo on and then a solid color cardstock to put the mounted pattern paper on. If you choose your colors right, this can create a very neat effect.
I have also used three or four colors of card stock to mount one photo. Often I'll do this on a title page, where I am only using one photo. Again, depending on your color choices, this has a very pretty look to it.
If you are using squares to mount your photos, try turning the square to make it diamond shaped. You can also use several diamonds, all in various colors to mount your photos.
For a more abstract look, try using three colors, all cut into squares and then turn them each a different way, all on top of each other. You end up with a mount of several points.
There are a few ways to turn these mounting squares into other shapes. I love the cutting tools that Creative Memories sells. They sell them in many different shapes. You simply trace the shape with a blade and you've got your mount. There is also a system called Coluzzle. This brand can be bought at most craft stores or through Leaving Prints distributors. Again, it's a plastic frame that you run a blade around to create your shape.
Once you begin playing with the many shapes, you can create all kinds of effects for your photo mounting. You can mount a square inside a larger circle and then put your photo on the square. You can put a small heart inside a large heart, and then mount your photo.
Have fun mounting your photos with various colors and shapes. Create unique albums and unique pages by using your own imagination and creativity to decide how you'll mount your photos.