Cooking lentils so they hold shape for salads instead of becoming soup

I want lentils for a cold salad with vegetables and vinaigrette, not a pot of soup. My last batch turned soft enough to collapse when I stirred it. I used ordinary brown lentils and boiled them vigorously while cooking other things. Is the biggest difference choosing firmer varieties, simmering gently and tasting early? I'd love a reliable method where they stay tender but distinct.