Agreed with @mitsuki-youko that it's all about practice. I would only add to take a hard look at how you do it, and see what you can improve in the way you draw.
Using myself as an example, I noticed I spent a few seconds looking up at the gesture (second monitor), then down to draw, so I rearranged my space in my tablet to have the reference at the left of my canvas. Now my eyes just need to flip slightly as I draw.
I also noticed I spent time "looking for brushes", so I adjusted my shortcuts to select the only two brushes I use, now I can switch between them with a single key press at my finger (I bought a keypad, pretty cheap, and helps keep shortcuts at hand.
Just a step back, analyze how you go at things, and see if you can adjust something. If it works, you keep doing it, if not, maybe try something else.
I really don't think Marc means tracing. Tracing in general is not well viewed, and though I believe it does have a place in learning, it easily becomes a crutch if practiced frequently.
What I personally do at time for copies is use a variation of a grid to keep proportions more accurate, something like this:
Here's an example I did recently:
But still, only for the basic construction, afterwards I remove the grid and try to copy through observation (which is why I ended up going outside of the rulers a bit, most noticeable in the hair)
That's only for when I'm doing copies, usually I'll try to use the original as just a reference.
Granted, many will prefer to just go by observation and do construction on their own, which is also great practice.
Overall keep in mind the objective is to "observe" and "absorb" what the artist is doing. How he works his lines, how he constructed and ties around the different elements like muscles, clothes and so on. Tracing will usually not get you this benefit, as you are basically painting over without much consideration for what goes into "building" the fundamentals used.
Hope that makes sense.
