make sure that all the rock is very snug against eachother. you dont want them toppling over and hitting the glass/acrylic of the tanks. once i know where i want the rock to sit, i try to wedge it between the other rocks, and tap it with the heal of my hand to make it snug.
some people actually peg their rock together (drilling 2 adjacent holes and putting a "peg" between them to join the rocks. the peg is usually 3/8" hard air line), some use water proof epoxy (i use aquamend to peg my coral to rock), and i've even seen the rock tied together with fishing string ( its actually almost invisible underwater).
some of the bigger snails (mexican turbo's and conch's) are known for knocking over rocks. good luck!