Oh, I actually feel sometimes I can’t achieve that. It’s pretty difficult to keep the freshness from sketch to finish - this is a widely debated/written about topic, in fact (I think maybe someone wrote about it in Muddy Colors at some point).
Personally, I guess I work in a way that makes my line work fairly clean, and I actually like my sketchy lines to be a bit visible in the final. I never ink anything. I try to pay attention to my line value, so even those wispy sketchy lines are pleasant to the eye. I’ve spoken before about how I colour my lines, which I feel also helps bring the whole thing together.
And actually, even though I love colour, I often like peoples’ sketches more than their finished work!
I guess I’d suggest working on your line value, so you can go from sketch to final without much editing. And do your best to colour/paint your work in a way that complements your lines. Not sure that’s much help, but I hope it’s something!