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!) 
`anmari has been spreading her infectious positivity throughout our community for over 6 years. Throughout this time Ana has been at the core of all things devious, passionately developing an eclectic gallery, helping organise devmeets, participating in chat events and also recently completed dedicating her time as a Community Volunteer. We are absolutely delighted to bestow the Deviousness Award for May 2013 to `anmari, congratulations! Read More
Sta.sh
Thank you, though