Below are some helpful tips for preparing for tryouts and to do well in tryouts:

 - Arrive early to tryouts
 - Get a good rest the night before
 - Bring a positive attitude and don't get discouraged
 - Eat properly beforehand
 - Come prepared, wear shin guard, cleats, socks covering shin guards, appropriate apparel, look like a player that knows what they are doing when it comes to soccer. LOOK THE PART!
 - Bring a size 4 soccer ball with name and phone number on it in case it gets misplaced.
 - Stretch before tryouts. Dynamic warmups are best. There are a lot of online resources.
 - Bring WATER! and plenty of it. Tryouts are often at the beginning of the summer when the temperatures are starting to climb for the summer.
 - In the weeks leading up to tryouts, get as many touches on the ball as you can. There are many online resources (YouTube is the easiest for finding drill for kids to work on). Get outside with a parent, sibling, or friend and get as many touches on the ball as you can. Work on your passes, shots, and dribbling as well as controlling the ball with your body and trapping the ball.

During Tryouts: GET NOTICED
 - Keep a positive attitude, listen to the coaches / trainers / evaluators
 - Be willing and determined
 - Coaches are often looking for defenders. If you are confident in your defensive skills, let the coach know. However, it's best to be honest and tell the coach what your strengths are including positions when asked. Coaches are also looking for players that are willing to play whichever position is needed so be flexible. At the U10 and U12 divisions, coaches should still be moving players around to different positions and not just have them play one position. The best positions are striker either. If you want to get far, don't limit yourself to just being a striker. Play the other positions well. I reserve the position of center midfielder for my best players. 
 - Work hard every moment. Coaches aren't always looking for the player that is fastest but often look to see which players finish strong and don't stop short. Coaches will often ask the players to run a lap or do some exercises and will instruct players they don't have to run their fastest but make sure to run all the way through.
 - Listen!
 - Communicate well
 - Be team player
 - Display confidence in your abilities

Don't get discouraged. Don't get discouraged if you don't do your best on the first day. Players frequently get noticed on their second day of performance. It's always a challenge during tryouts for coaches and trainers to identify the players that should be playing at the level they are trying out for. There is often plenty of talent and sometimes kids get turned away that would have done well on the teams. Don't get discouraged if you don't get selected. Try out again the next year if you're not selected. Again, stay positive. Sometimes coaches later get openings and will reach back out to players that stood out to them at tryouts but at the time they didn't have an opening.

GET NOTICED! For that very reason mentioned above, get noticed and in a positive way. Make every shot, every pass, every dribble, every touch on the ball the best you can and keep working hard. Thank the coaches. Don't ask if you're going to make the team, show them that you should make the team.

Once you make the team, keep working hard. Your participation on the team is always a tryout for the position you want and a chance to show the coach you should be selected again the following year. I've often instructed my kids to show that you deserve to be on the team. Then when they get on the team, I tell them they now need to show that they deserve to play the position they want.

Additional resources:
Soccer Tryouts: 15 Helpful Tips (Stand Out / Make the Team)