If you want to learn to focus in manual mode and you really don't want to "cheat" it helps if you get a tape measure or a measuring device of some kind. One way to do this: measure out a set distance, like say 5 or 10 feet, focus on that, then try moving around to learn how to estimate distance. Once you learn where your "x foot mark" is, stay there and bracket your aperture (so shoot at f/1.8, 4, 8, 16, etc.) to see what it does to your focal plane.
It might seem like a lot of work but this will be really helpful in the long run. You'll be able to estimate a good shooting distance and really know where your focal plane begins and ends. If you do this with something like a "nifty 50" and really train your eye, you will have an intuitive feel for your lens. You will get way more consistent results than somebody who has a 3-300 just "puts it in the middle," guesses, and relies on AF to make focusing decisions for them.
G'luck and congrats on moving up to the DSLR