I'm very motivated by the recent DD for my Tiny Tips. I've been asked for more water explanations, and I've taken them all into consideration, but this turorial was already in the pipeline.
I strongly suggest to stay away from the stereotypical overly full lips if you're aiming to paint a pretty girl. Marta Dahlig has done some very good tutorials for the ImagineFX on the topic of diverse forms. Aside from the much-cited character the forms of lips can convey, also keep in mind that faces have expressions, and it makes a great difference if your characters are smiling with just one corner of their mouth, biting their lower lip, or maybe hold something between their teeth (just no cigarettes, please. Smoking is bad for you, don't promote it.)
As always, looking at lips will help painting them (fortunately most people have a mouth, so a mirror will help a lot).
"Ranarh's tiny tips" is a collection of iddy-biddy tutorials, each containing small bites of advice anyone can chew, aimed at beginner's level artists. They are not software-dependent. If there's a problem you have, something you feel someone should finally explain, or are just curious what I have to say on a particular artistic issue, feel free to tell me.
You'll find the whole Tiny Tips Collection, plus some full-grown walkthroughs in my tutorial section
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