I don't know if this has been mentioned already or not, but I learned this great trick from Jennifer McGuire last week using Liquid Pearls (comes in a bottle, like Stickles, but is pearl)...
Squeeze out a little Liquid Pearl onto a scrap, take a paint brush or water brush and just "paint" your area/project with the watered down Liquid Pearl! Perfect amount of shimmer, yet totally transparent so that the color of your image shows through. I was amazed at how perfectly it worked! Hope it helps.