There was a picture posted of a three tier heart cake that was covered in red crystals. I thought how beautiful that would be for a wedding. How would I get the sanding sugar to stick to the fondant particularly on the sides.

Not sure - haven't tried so just guessing - maybe using an egg white wash or a light coating of piping gel??

anything sticky should work (thats edible), piping gel, corn syrup, gum glue. do a practice test on the back of the cake first to make sure.

Hi it was my cake I believe your speaking about. I covered it in fondant and let it sit for about 1 1/2 hours and then I slathered it with thinned down piping gel and covered it over and over with the red and pink sanding sugars. Worked well and the gel dried well and really held on to the sugars well also.

I brushed Bakery Crafts glitter gel on my fondant cake before adding the sanding sugar. It really added a lot of sparkle.
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1168552

Glitter gel: http://www.pastrysampler.com/Products/glitter_gel.htm