Bury most of the rocks in the slope. Frequently, in a rock garden, as little as ten percent of a rock's volume is exposed. Position rocks so the most interesting weathered face shows. Once you have an attractive design, don't spoil it by adding a random collection of other types of stones, painted rocks, or novelty pottery items. The point of a rock garden is to bring nature, not theater, to your garden.
Scale is the most important factor when selecting plants.
They should neither dominate nor be lost in their setting. Plants that grow too large will hide the rocks while dwarf plants can be hidden among the rocks. In general, low creeping plants blend in well with rock garden designs. Dwarf and creeping varieties of junipers, creeping cotoneaster, mountain pinks, dwarf rhododendrons, and low and creeping perennials are good choices. If you have a location that receives at least half a day of sun, grow herbs and sedums, too.