There are two basic aspects to steering a crewshell well: choosing a point to steer for and holding to it, and choosing a line. In a bowloader, the best way to choose the point is visually to align the forward point of the coaming (that's the higher part of the gunwale, the part which comes to a point over the foredeck) with the bowball, and align both with a point on a far shore to steer towards. Aligning the point of the coaming with the bowball, instead of just the bowball with the point, gives you a little extra accuracy, and also helps keeps you lying still and in the center of the boat. In a sternloader, this becomes a little trickier in that you can't actually see your own bowball, nor the point you are steering for. The solution is either to pick an object high enough up that it is visible over your rowers' heads, and to align with that, or to pick two points visible on either side of your rowers, and to keep the shell pointed between them. On a buoyed 2000 meter course in an 8+ (for example), a useful method is to use the perspective of the buoy lines so that they seem to converge into your four or five seat's ears equally on either side.
The limitation of steering to a point is that -- even if you hold your bow exactly on the point -- wind or current can still blow you off course so that the shell describes a curve (mathematically plottable according to a Fibonacci progression, for those who are interested in such things) as it moves. Also, in the head race season, or even while practicing on a curving river, a straight line may not be the one you need. You may be pointed straight at a bridge arch, or at the next curve in the river, but that won't guarantee that your shell is in the best angle to negotiate that arch or curve when it gets there.
This is where the next step of steering -- choosing a line -- is so important. Choosing a good line is what makes good coxswains really valuable to their crews. It consists of being able to "see" where the shell will be at every point along its course and to know in advance what angle it needs to have at those points. Even better, once you get the hang of choosing a line, that skill will help you predict the line that opposing coxswains -- or coxswains on your own team -- are taking. This will not only help you avoid collisions during practice, but could give you a competitive advantage in a head race.
Whether you "see" the line as an imaginary mark along the water (sort of like the first down mark effect that television broadcasts of football games use) or picture your boat at various stages along it (say, every twenty-thirty meters in momentary flashes), which is the method I find most effective -- or some other method you come up with, it helps a lot. (It's best not to mention that you're seeing "imaginary boats" to your rowers; they might get the wrong idea.) Picturing your boat's future course in your mind also helps cultivate a general habit of anticipating -- a valued attribute in coxswains. "Where do I want to be, and what angle do I want to have when I get there?" It allows you to set up properly for turns, or docking, far enough in advance to handle the necessary maneuvers smoothly and efficiently. Another way of thinking about it is that you are "placing" the shell where it needs to be, rather than just steering it.
If the course is one you can walk in advance, it is very helpful to do so while picturing your boat at various points along it, and the mental exercise can pay dividends later in the race.
It takes a while to get the feel of this approach, but as you do, you will feel your steering become more natural (the shell will almost seem to place itself). Complicated maneuvers will seem more straightforward when pictured ahead of time, and you will feel more in control.