To keep the avocado from sliding around the plate, cut a thin sliver of skin from the rounded side before filling the avocados. That flat portion will help keep the avocado steady on the plate. Variation: For a more dramatic presentation (one more suitable for a sit-down dinner party than a weeknight supper, perhaps) present the salad as a "stack," that is, layer the ingredients in a ring mold (similar to a biscuit cutter without the handle). Instead of serving the salad in the pitted avocado, peel and pit the avocados, then cut them into 3/4-inch dice. Toss them with a little lime juice to keep them from turning dark. Make the crab salad. Place the ring mold over the center of the plate and put the equivalent of half a diced avocado in an even layer in the mold. Tamp it down gently. Top with one-sixth of the crab salad and tamp down gently. Lift the mold carefully and the ingredients will remain in place on the plate.