Very Important - leave her alone! If I see a parent trying to coach their DD during a tryout it is an immediate red flag. Parents need to sit in their chairs and stay quiet during tryouts.
Make certain that she wears her ball pants and a normal t-shirt. No mid rift shirts and short shorts. Her hair needs to be back, before she gets there.
You would be surprised how many girls show up - and find out they left their glove in mom's car, etc.
Bring a igloo/thermos of water. Not a small bottle that you picked up at the gas station. It isn't enough.
Jog every where on the diamond, smile, compliment your competitors, and learn some names. I like it when someone asks what I like to be called, if I have forgotten to tell you. It would rare for someone to be Mr. _____, but if he is a local teacher, that might be the case.
Excellent advise so far. I agree that hustling, listening, interacting and giving your physical all are a given. Make sure your DD does not overlook the mental aspect as well. At every TB tryout I have held I take time with the players to ask several questions. When I have several players that are physically very close the answers to the questions are often the tie breaker:
Q: Why do you want to join this team?
Answers that don't help get you on the team:
A: My dad says it is the closest practice to our house
A: I don't know, I heard you needed a (insert position here) so I thought I would tryout
A: I am trying out for a bunch of teams, just covering all my bases.
Answers that help get you on the team (these fall under the previous comment that kiss a$$ helps)
A: I am looking for a competitive team that can use my talents and skills
A: I saw/played against your team and want to be part of this organization
A: Remember I pitched that no hitter against your team in XYZ tournament championship game (Yeah I remember and it still burns).
Also make sure the T-shirt isn't one from a rival travel team Tournament t-shirts are good choices.
-W
If she is trying out for the same org. she was with the previous year, what about wearing their jersey? How about all-star jersy from her rec. league? Should she wear pants in this heat, or will shorts be fine?
Pants and knee pads are advisable. I'd stay away from wearing their jersey, but an org t-shirt is fine. I'd stay away from all-star jersey's too, as a coach, I find them to be a bit of a turn-off, as the kids that are really proud of them are usually lesser players and the kids that take a "ho-hum" approach to their all star jersey's usually prove to have more ability. Just my experience though, and I'm threading a needle. Obviously if your kid can hit and hustle, she'll have no problem.
Can't stress hitting. If she can hit they'll find a place to hide her. Running is the next skill I look at. Speed wins.
-W