Blu-Tak is one solution, but for about one-tenth the price you can buy essentially the same stuff at Office Depot or Office Max. The product to look for is Elmer's "Tac 'n' Stik" reusable adhesive. I've used it for years (currently using it under my center speaker and rear surrounds) -- it works every bit as well as Blu Tak, and sells for 99 cents a package. The adhesive is scored into 5 sections, and four half-sections, each rolled into balls, are enough for each corner of the speaker.