It is most important to turn accurately. Try to minimize the amount your cubes have to corner cut, and have wrist turns be efficient and snappy (not overturning). When drilling, slow down your turning as much as you need, then slowly increase turning speed while maintaining the same level of accuracy.
You also need to be sure that you are using all necessary fingertrick techniques where appropriate. Pinch turns are especially important, and I see missing/underutilized for a lot of sub20 cubers. Cutting down on regrips is a big deal. And fingertricks like D2 flicks are things you actually have to learn; don't try to find workarounds for the standard stuff.
Ultimately you just need to drill a lot. (Back when I cubed, I spent a lot more time on algorithms than actual solving.)