An unnecessarily long reply:

Ummm well I guess first off I realized about a year ago that you can’t just practice drawing designs. Each outfit doesn’t always look better than the last, sadly, like most drawings—so actually hours and hours of simple sketching really won’t get you anywhere.

I’m no SRS HARDCORE CONCEPT ARTIST BADASS DESIGNER of any sort (yet, hopefully) but I put in a hefty amount of research for all the character designs I make. Usually I refer to historical fashion as a ‘basis’ for every single outfit—because everything is basically a reference to history. It stands as the ‘outline’ for all fashion (whether fantastical or present-day) and from hipster feral boy jackets to spacesuits to sometimes-impractical video game armor, all share common historical roots—so I study that, first. Then I add in details. Here’s a sort of ‘process’ for my outfit designwork:

1. a bit of historical research, and a background check on the character I design for—what point in history can I trace it to? culture? (or mix of?) and what kind of environment are they from? (of course, this can all be avoided if you’re just making outfits for fun)
2. then I make a basic outline of the outfit in simple lines and geometric shapes, no details or accessories
3. I then go through another interlude of brief consideration on the character’s economic class, job, and personality: then I add in accessories and details. is the character going to wear a fancy layered robe? or a simple toga? do they carry weapons? jewelry?
4. afterwards I add that on, but following that, I throw the design away to either work on character design or something else: key point here is, I wait a bit before coming back to it (an opportune time to make another design! fuck yeah!)
5. several weeks, or a month or so later, I would return to that outfit and adjust it around a bit more. usually by then I would figure out how to improve it. by now, I would either (a) make a few tweaks or adjustments, that declare it as ‘FINAL’ and I don’t know what comes out of that orrr (b) remake everyfuckingthing (and so continues the cycle)

TO IMPROVE: like I mentioned, practicing does help but only so much. Design focuses more on creative ability, and studying (but it’s fun). Mentioned before: I refer to historical designs, and what was popular at the time, and how it related to environment, society, culture, and function. I also play video games to observe character designs. Of course, just looking at them doesn’t help—you have to notice common themes throughout costumery. What makes something attractive? Why are some outfits still viewed as beautiful despite centuries (or millenniums) of fashion development, while others just make you want to vomit?

It’s kind of inexplainable, but more than anything else it’s paying attention to repetitive patterns in design themes—then applying them in the most beneficial way to your own outfit. Try out designing with different colors, and mixing in different themes (asian/aztec-mixed outfits? on my to-design list!)

STUFF TO CONSIDER:

• huge huge huge, cannot stress this enough—BACKGROUND CHECK ON THE CHARACTER. Consider everything. Fashion always reflects on the wearer in not only personality but also culture (sorry, being repetitive here). Additionally, practical clothing is always an effective element of outfit design. It makes it more believable, and ‘dynamic.’ Do not throw something on a character just because it looks good
• another point on function: is your character a fighter? if so, throw in some belts to hold knives and overstuffed pouches for adventurers. magic user? NERD SYMBOLS. political leader? badges and shit, or toss in a tacky crown. also what kind of climate do they live in?
• differentiate between characters of ‘high’ and ‘low’ detail. I usually keep high-detail outfit characters for those of higher rank—at the opposite end of the spectrum are the typical low-detail characters of religious figures, and peasants. and when dealing with detail, remember that repetitive patterns and shapes unify the design, and leading to the next point
• UNITY! is your design unified in terms of colors, shapes, patterns, and ‘areas of detail’? I tend to stick with an overall color scheme of 2-3 colors (with different shades in-between). I keep patterns a common element, too, as they are indicative of textures (leading to background information culture, etc. yadda yadda). And of course: a common repetitive shape along with everything else. REPETITION!!
• does your character have a balanced, symmetric design or something asymmetrical? I tend to favor unbalanced designs as they are more dynamic and interesting to look at (but more difficult to master and draw) this is created with the traditional belt-across-the-chest technique infamous in video games, but there are tons of other stuff you can include too like capes, differences in sleeve lengths, and whole tons of other stuff in general!

I can’t think of any other stuff to toss in here, but hopefully this helps! Really these are all the things I consider with designing outfits  (there’s probably a bit more that I’ve forgotten, though)

Good luck!