There are two major problems in sweater making (design.) First, figure out how to make the shape you want out of knitting, and second to place some sort of ornamental feature (pattern) in the knitting without disrupting the shape.
Shaping isn't easy, but it's solvable. Once you figure out how to make the shapes you want, it's just a matter of implementing a known process. Shaping becomes trivial.
The second problem, the design, is the really clever part.
Fitting patterns and embellishments onto a shape, just isn't solvable, and never becomes trivial. There are tricks, and practices makes it easier, but the possibilities are truly endless and there's no reason to make any two sweaters exactly the same. 
The patterns and embellishments can be pretty broadly defined: cables, colorowork, other texture patterns, and so forth.
In my own knitting, I have taken to focusing almost entirely on the second problem. I have a basic sweater form that works really well for me, and each sweater explores a different combination of patterns.
My approach is as follows:
Divide the sweater into quadrants and plan a single quadrant of the sweater. Repeat this pattern over the entire sweater. This automatically centers the pattern on the sweater.
Be flexible with the number of stitches but not too flexible. Also remember to account for a "middle" and "end" stitch which may not be repeated on every quarter.
Think of birds eye view of the design. This means thinking about sweater design as a collection of pattern columns.
Use patterns at either side of the sweater both for a nice effect and to "bound" the patterns. This can be helpful in controlling the number of stitches.
Unless you plan to knit your sweater horizontally, plan your sweater virtually even if you have horizontal patterns. It's crucial to center patterns that run horizontally across the sweater. Thinking vertically is the easiest way to do this visually for all kinds of patterns.
If you're using only one pattern that's fewer than 10-15 stitches, you may be able to just make sure that your pattern divides in the total number of stitches in the sweater, but that's a much less common problem.
Beyond that, it's all trial, error, and practice. Onward and Upward!
|||There may be some exceptions, but generally.|