In a large, heavy saucepan, heat the milk (and cream, if you're using it) and salt over medium-high heat until it's steaming - if you have a thermometer, it should read 190F. Stir it occasionally to keep it from scorching on the bottom.
Remove the pot from the heat and gently stir in the lemon juice, only stirring a couple times. Let it sit undisturbed for 5-10 minutes.
Line a colander or sieve with a double thickness of cheesecloth and set it in a large bowl; pour in the curds and whey. (If the whey is too deep and the colander is sitting in it, you can pour it off into another bowl.) Let the ricotta sit for an hour or so - the longer it drains, the drier it will be. Place a plate on top of it if you want to press out as much whey as possible.