It took me a while but I finally got reasonable at this. Things I learned were.

Arm needs to flow from shoulder for both top and bottom lines.

Always be looking a the next turn back to judge your curveback right. It helps keep your vertical alignment.

And from Uncomfortable's feedback to me, remember perspective. In particular the lines should overlap and compress more as it comes towards you.

But like most things practice. Look at what doesn't look right. Isolate that and practice it. Then try again.