When making a traditional pasta and tomato sauce recipe, there are two techniques I've come across on various cooking blogs which work well to prevent puddles of water from forming on the plate when the pasta is plated. The first tip is to thoroughly drain the pasta then return it to the still warm pot and let it rest until the steam stops rising off of it, before dressing it with sauce. This will let much of the excess water evaporate off. The other method is to drain the pasta thoroughly and return the pasta to the still warm pot and dress it with the sauce immediately, then put a lid on the pot and let it rest for several minutes. This will allow the pasta to soak up the residual liquid a bit before plating.
For making a homemade mac and cheese sauce, it's been suggested to add a little sodium citrate to the cheese sauce to make the cheese melt more evenly, and prevent any separation or graniness in the sauce. However it's difficult to find sodium citrate in the store, so you usually have to order it online (and it should not be confused with powdered citric acid, which is not the same). Also, if you use too much of it, it can impart a salty metallic taste to the sauce which unpleasant.
A better option I've come across, which I was admittedly reluctant to try since it sort of seemed like a "cheat" like using Velveeta, is to add a few slices of plain old American Cheese to your bechamel sauce and letting it melt in completely before adding your other cheeses. I get some Land O Lakes brand American Cheese from our deli, and use that along with some hand shredded sharp cheddar, and it works like a charm. American Cheese is typically just scraps of cheddar and colby cheese that have been emulsified with water, so the cheese melts better. The added benefit I noticed was the sauce actually tasted more "cheesy" too - which was a common problem I would encounter when making a cheese sauce from scratch. Often times the bechamel sauce alone would cause the sauce to taste too bland.