Edit: Adding some more to help make this tut more unique and helpful xDDedicated to all mouse artists who think they can't do what tablet users can
Play around with the pen tool and brush settings to get the desired look you like.
**This method works just fine with tablets as well
Should work with versions 7+.Watch video tutorial on Youtube: [link]
How to do it on SAI:
GIMP (looks similar):
Image from Tut: (Video process linked in artist's description)(If you can find equivalent tutorials in other programs, let me know and I'll include them in this description!)