There is this great tutorial by GimpGuru for sharpening pictures without amplifying the inherent noises. However, the first time when I followed his instructions, I fumbled: I didn't get my sharpened picture; I got confused instead.
But with some perseverance, I succeeded in the end.
I thought of streamlining his tutorial as I know that I will be using it often. But a day later, I found an even better tutorial/technique, '"Smart" Sharpening, Redux', also from GimpGuru. I particularly like the ability in this second technique in adjusting and observing the sharpening level, WYSIWYG-style, by merely sliding the "Opacity" slider left or right in the "Levels" dialog window.
In case if you are wondering, I fumbled on the second tutorial too the first time round - I got confused just like the first.
Tutorial - Made Simple
Below are my simplified instructions for the second method by GimpGuru. Have fun! I am using GIMP version 2.6.9, by the way.