LAYERS!
As Mugenjohncel pointed out above, layers are awesome and versatile. I draw a quick gesture layer, lower the opacity and draw a cleaner version on top on another layer, and repeat as many times as necessary, each time everything becoming easier and faster.
It also allows me to focus on sprite POSES and imparting good character and energy before ever worrying about clothing. Which I add on top using - you guessed it - another layer. Yes, every character I draw starts buck naked, because I'm not always sure how revealing future outfits may be, then I can reuse that pose base to add all the different outfits on top like a paper doll.
I also make all the outfits composed of different layers so I can change colors or details very quickly and easily without redrawing or repainting anything. It is a trick I picked up when I was working with film special effects - we rendered out everything in passes so we could tweak details to perfection without being forced to waste time and money re-rendering a shot. Then we could composite everything for the final image, and if the client decided they wanted an apple green instead of red it was a 5 minute fix rather than a 5 hour wait for another scene render.
Oh, and my other artist trick is that my "picture morgue" of reference is MASSIVE, and well-organized. I've got 400 GBs of reference images at high resolution, organized in such a way I can find what I need for any project very quickly. It is organized by subject AND time period. I've even got a folder of literal morgue reference, divided by time period. (Doing research for movies and games can get . . . bad at times. I once had to research period correct child burials and coffins for a game.
Re-opened coffins, I might add. 
That'll give you nightmares. I've got parts of my picture morgue that I avoid like a small child avoids their dark closet . . . .) I've even got a reference folder for cinematic angles divided into what emotions and responses they are designed to evoke. Period-authentic Victorian Halloween Costumes? I've got those too . . . crazy prepared is what I'm saying. Before computers I used file folders and cabinets with magazine clippings for my picture morgue . . . .
Someone might mistake my human reference folders for a porn collection, except people of all ages and genders are represented, even the really old wrinkled ones! (Well - I guess someone might think I have REALLY weird tastes . . . .

) You know you went to a liberal school when the instructors handed out 20 GB of naked people to the students for art. The photos are such high resolution you can zoom in to see individual pores and spots ON freckles. Since I did 3D art and film effects we sometimes needed the actual photos to create textures.
So there are my two secrets - layers for modular design and modification and crazy amounts of reference.