Measure the vinegar into a small bowl. Crush the garlic clove and add it to the vinegar, along with 1/2 teaspoon salt. After 10 minutes or so, whisk in the olive oil and a little freshly milled pepper. Taste and adjust the seasoning. Make the vinaigrette no more than a couple of hours before serving.
Pick over the salad greens, discarding any tough outer leaves, or any that are wilted or blemished. (If you are using young head lettuces, trim the root ends with a sharp paring knife to free the leaves.) Fill a large basin with cold water and gently submerge the greens, allowing sand and grit to sink to the bottom. Lift the lettuces from the water and drain them in a colander, then dry them in a salad spinner. Wrap the leaves in clean cotton toweling and refrigerater in an airtight container.
To serve, put the greens in a wide salad bowl and season with a small pinch of salt. Remove the garlic clove, whisk the vinaigrette, toss the greens lightly with just enough dressing to make the leaves glisten. (Your hands make the best salad-tossing tools.) Serve immediately with garlic toast or croutons.