It might seem counter-intuitive to add more liquid when your sauce is already too thin, but the starch in the pasta water actually helps the sauce to "tighten" and thicken up. The result is a silkier and creamier sauce, sans actual cream.
Just before draining your pasta, scoop out a bit of the cooking water. When your sauce is ready, add about a fourth cup of the pasta water and let it simmer for a few minutes until it thickens and gains body. You can add a little more pasta water if you think your sauce needs it, but be careful not to add too much or your sauce really will be soupy!
Give it a try next time you make a pasta dish!