I usually use Mandalinn's method of drawing a line to divide the food into however many portions it is supposed to be.
The only food this doesn't really work well with is soups and stews. Those I just eyeball and try to err on the conservative side the first night so that I'm not gypping us the second night (there's two of us, so if something makes four portions, we have it again for dinner the next night). We use the same soup bowls all the time and I make enough soup/stews that I know about how high to fill them to equal one portion. I figure that so long as I keep both of our portions about equal, the calories all balance out if I eat a little less the first night and a little more the second night. Because I'm always adding veggies and other ingredients, sometimes when I've measured out all the soup portions (I typically store leftovers in single-serving containers using my standard soup bowl to measure them out), I'll discover that it came out to five portions instead of four. I count this as a pleasant surprise, adjust my calories for the day, sometimes eat something else if the calories came in really low and I'm still hungry, and note on the recipe that it is five portions instead of four.
If I'm feeling really anal about it, I will weigh and measure the portions. Usually the way I do this is by first weighing the entire amount of food the recipe made in grams. Then I divide that amount by however many servings the recipe is supposed to make, then measure out the individual portions. Alternatively, I'll pick out four containers (assuming the recipe made four portions) that all weigh the same and divide the food into each, weighing each to make sure they all weigh the same amount. I'm more likely to use the second method when I'm not eating any of the portions right away. My SO has suggested that this behavior is excessively obsessive

, so I only do it if 1) in the case of the first method, I have a bowl or pan left from preparing the meal that I can dump all the food into to weigh it (I refuse to dirty an extra pan to weigh my food) and 2) My SO isn't around or likely to walk into the kitchen while I'm doing it.