Homemade French salad dressing, or vinaigrette, is better and cheaper than almost anything you can buy. A combination of oil and vinegar ("vinaigrette" is the diminutive of vinaigre, the French word for vinegar), it can be flavored with fresh herbs and minced garlic or shallot.
Most recipes call for gradually whisking the oil into the vinegar until the two ingredients are emulsified. This results in a fairly stable vinaigrette, meaning that it will stay combined for a while (though eventually the oil and the vinegar will separate again). But we usually just shake the ingredients in a screw-top jar instead. The dressing won't stay blended as long, but it certainly simplifies the process.
FOR EXCELLENT SALAD
Dry your lettuces well
Spin in a salad spinner or dry carefully between towels, or else you'll have a bowl of sogginess.
Be sparing with dressing
Use less than you think you need (start with 2 tbsp. for 10 cups of salad) to help keep salad crisp. Once you've tossed and tasted, you can always add more.
Toss well but gently
Use your clean hands; they're your best tools for coating leaves thoroughly without bruising them.
Use a big bowl at least twice as big as the salad. Plenty of room makes it easier to toss the contents.
Dress at the last minute
For the crispest salad, dress and toss right before serving.