Here's my tips:
1. look your best- hey, if you're a cute girl, don't cover that up. Not slutty but professionally attractive. Do your hair nice. I have a mid-length, short sleeved, collared dress with a fun print and tight belt I light to wear with shorter heels. very conservative, but nice. Shows you care about how you look.
2. Be prepared. have portfolio, business cards, and research beforehand who you're meeting with and who they represent. be prepared to tell them how YOU can help them, and how they can't get your talent from anyone else.
3. Know your inspirations. seriously, nothing looks worse than "who do you look at?" "ummmm....I'm my own inspiration.." NONOOOO My answer is usually: "Alex toth, Paul Felix, Gustav Teneggren, Oli Josman, etc" List both contemporary and older illustrators.
4. Zen. I was once called out for being too excited about something, because I was nervous. People need to know you work well in a team environment. Be cool. and this is what helps me with that...
...drink a beer (or take a shot) before the interview. seriously.