POPSUGAR: What defines grown-up, sophisticated hair for a late-20-something woman to you?

Sherry Heart: I remember in my late 20s being so concerned about turning 30. Where was I at in my career? Where was I headed? Those questions plague your brain when you're headed for the big 3-0. I think many women get to their late 20s and decide to make a complete change in their look in order to appear more together and serious. I have seen girls cut their hair short for a more professional look.

PS: What defines more youthful hair for a 20-something woman?

SH: When you're in your 20s, you pretty much have license to be whimsical and varying in your hair choices. When I was that age, I had many different colors, styles, extensions, and hairdos. Young women are braver and have more freedom during their 20s than when they get older and think they have to look a certain way for their careers. I always encourage any 25-year-old to try a different hair color or hairdo, because she most likely will want to go more conservative as she gets older.

PS: What are mistakes every 20-something woman makes when it comes to hair?

SH: Many women in that age group get into experimenting with their hair and accidentally overprocess it. Before they know it, they have dry, damaged hair. Always have a great deep conditioner on hand.

PS: What tips do you have for someone trying to make his or her hair seem more grown-up? What styles should they try?

SH: Keeping your hair neater and carefully styled can only help you to obtain your professional goals at that age. Try straightening your hair in a sleek ponytail. For cut, a nice below-the-chin bob most always adds a professional edge. Color depends on the person's skin tone. Consult a color specialist, and whatever color you choose, have a professional do it. Bad color is never attractive.

PS: What products will help you achieve that more stylized 'do?

SH: Right now, I am in love with Sachajuan Ocean Mist Texturizing Spray [4] ($28). It's great for adding a bit of separation and texture to the hair.

PS: And what are great hot tools to have?

SH: Always have a good straightening iron handy — remember, you get what you pay for when it comes to straightening irons. I love and am currently using the GHD Dashing Diva Digital Titanium Styler [5] ($125). I even use it to put a slight wave in the hair by rolling it down as I straighten.

PS: What if you're superlazy? How can you get good-looking grown-up hair easily and quickly?

SH: Slick your hair back in a tight, smooth ponytail and place it in the middle of the back of your head. Mist your ponytail with water, and then apply a thin layer of pomade to the hair and smooth it out with a brush. Next, twist the ponytail with your hand and then wrap it around a hair binder [hair tie] like Scunci No Slip Grips [6] ($3) in a circular motion. Pin the end in a disguising fashion, and voilà! You have a quick, easy, mature, professional hairdo.

PS: How did you transition Shoshanna's and the rest of the characters [7]' hair to be more grown-up?

SH: This season, we see Shoshanna continuing to evolve from the quirky character she was in the first season into more of an adult. She's realizing that college is going to come to an end someday and that she needs to think about getting a job. So she's trying to present herself in a more professional way.

Hannah is trying the full-time writing thing, and when I think about her character, I imagine that she can't be bothered to do anything complicated with her hair other than maybe to stick a barrette in her bangs to get them out of her face.

Marnie still wants — in my opinion — to appear to the world as if she has herself all put together perfectly. So, I try to keep her hair looking done up and finished.

Jessa is the gypsy of the group with a bohemian style. I like to imagine her character would treat her hair as a work of art, so she gets lots of different, fun hairdos.

PS: How have you changed Shoshanna's hairstyles this year?
Shoshanna had the most grown-up look of the season. She cut her hair into a bob, and I did some really cute vintage-inspired looks on her.

PS: What are some polished looks you recommend for a date?
SH: For a date, I think it's nice to wear the hair down and wavy. Use a wide-toothed comb, and run it up the hair once or twice to get a tousled, teased look. It's very girlie and sexy.

PS: And for weekend brunch?
SH: For an early afternoon brunch with no fuss, I love tousled hair thrown up on top of your head. Its can look so fresh and easygoing.

PS: What do you wish you knew about hair care in your 20s?
SH: In my 20s, I worked in a hair salon that did a lot of hair extensions. I wish I had learned more, and different, methods of installing the extensions. My favorites are the clip-in wefts. I use them all the time in my film work — either extending an actor's hair or making it thicker.

Watch Sherry's brilliant work on HBO's Girls Sunday nights at 9 p.m. EST.