1) Foam dots - adding foam dots to the backs of your embellishments are a great way to add dimension to your pages. I usually add them to any "flat" embellishments such as title spots and punched flowers.
2) Pop-up flowers - creating pop-up layered flowers are another quick and easy way to add dimension and interest. With a few basic tools you can create coordinating embellishments to go with any project you're working on. See my video tutorial posted here.
3) Double mat - giving your photos a double mat is a nice way to give your pages a very clean, very professional look. I do this a lot on pages that I plan to frame because when you frame a page it has to be relatively "flat" and not too bulky (unless you're using a shadow box, which I don't normally use). I usually put my 12 x 12 pages in a regular scrapbook or album frame for a few months before rotating them into the actual scrapbook album.
There you have it, 3 simple ways to create dimension on your pages. Care to leave a comment about how you've used one or more of these suggestions?