I've found that so much of what gives shading depth and interest is (bear with me) eyes and coloured lineart!! find a good way to colour a characters' eyes - it pays off, believe me >:3c Often an artpiece will look soulless (even with complete shading) till the above is applied. Same for coloured lineart - it can be a hurdle at times but it makes the shading pop sm

additionally, for softer/casual shading I try not to stray too much from the "source material" - the colour that appears most on the character! (e.g. typically in my case a yellow character will have orange shading as a base.)
If the character is warm-toned, I like to layer on top of this a blue (which is actually reddish gray in disguise, such as this) which has a super cool effect!! if the character is requiring of blue tones, however, I'll move to the blue end of the spectrum. And if the character is blue, I avoid the double layer altogether X)

but remember to avoid overshading!! If there's a point when you zoom out and the artwork is completely unintelligible, that's likely an indicator that there's too much going on. I may not be an ace at rendering but I love my minimalism regardless \^.^/

I hope these were helpful even the tiniest bit Q . Q!