First thing you might want to try: grab a stitch dictionary. Make samples of sts you like. Work up gauge swatches of the orig (plain) st, and compare to the new. Same size? Bigger, smaller? Does the repeat necessitate changing the number of sts you need? Work samples of same st using diff ndl sz. Certain sts look better larger rather than smaller. And vice versa. You might look at any of the sampler-type afghans which offer the chance to do many diff stitch ideas. You'll most likely find you're drawn to certain looks and also will get a feel for where certain ideas work better than others. (Seed st makes a nice clean afghan or scarf border, or buttonband, but you might not want to do the entire body of a sweater in it.)
Some years ago a designer displayed a teeny, tiny sweater (almost doll size) at local LYS....done in variegated pastels, hand-painted buttons...using various mitered st. I was beyond intrigued. However, not something I'd ever make. But recently I did a variegated rug for my kitchen, using some of the same design concepts.
Oft times, it's a matter of personal preference. Trial and error. There are books on designing garments, but deciding which st patterns to incorporate is only limited by your creativity (lots of practice....and a scrutinizing eye!).