Mac n cheese is not easy. Perhaps the difficultly of making a good homemade mac n cheese accounts for the popularity of boxed mac n cheese. There is a lot to consider. First, you want to choose a macaroni that will trap a lot of cheese. Hence, the people at Kraft and Velveeta use hollow, crooked noodles. I believe the Velveeta shells are also ridged. Ridges increase the amount of surface area of the noodle, thereby increasing the amount of cheese that can adhere to the noodle. So choose the right noodle.
Next is the cheese sauce. Don't use low fat cheeses. What's the point? Mac n cheese is not low fat. You'll just have to get over that. Your cheese sauce will begin with a bechemel sauce, so you'll already be using gobs of fat (butter and oil) and whole milk before you even begin adding the shredded (or small diced) cheeses. (Some of the good melty cheeses are not suitable for grating, so you'll need to get out your knife and dice them into suitably small cubes.) Right, how in the hell do you make bechemel?
If you don't know how to make bechemel, you can easily find out. I am pretty much out of steam right now (I defended my Master's Thesis yesterday and got somewhat bombed last night, and I haven't had breakfast yet today), but I will tell you this: by volume, cook one part flour in one part fat (a mixture of butter and olive oil works nicely for this). When the flour is fully cooked (smell it, it should smell toasty) (and look at it, it should look "blond"), then you can incorporate your milk. Whisk constantly both when you are cooking the flour in the fat and when you are incorporating the milk. Because you will be adding cheese to your bechemel, you will want to make it much thinner than you would for a normal bechemel. When your bechemel is appropriately thin, begin slowly incorporating your cheeses. Don't throw them all in at once. Put the flame on low and slowly whisk them in. When all the cheese is totally melted, check the thickness of the sauce. If it's too thick, you can incorporate some more milk. If it's too thin, well, that problem is somewhat harder to correct, especially if you're out of cheese. Some of you may want to use a recipe for this, so you know how much cheese to buy and how much flour, oil, milk to use. If you don't have a sense for this, do use a recipe.
Finally, toss your macaroni with your cheese sauce and eat at once. Baked mac n cheese is popular, but baking is also the major culprit behind dry mac n cheese. A homemade, dry mac n cheese is not superior to a boxed mac n cheese just because it's homemade. I also like to add chopped up bits of various spicy pickled vegetables. The acid cuts the creaminess of the cheese really nicely and probably aids in your digestion of it.