Hi and welcome to the site.
Make sure the try-out announcement is posted everywhere and every way possible and make it inviting to those who have perhaps been out of the game for a while.
Talk to the kids that you know ARE coming out and encourage them to encourage those you have in mind. Positive peer pressure is a powerful thing.
Sounds like you have some particular kids in mind who you may know. Call them directly and encourage them. Communicate to them what it is that inspired you to post this. If you think it will be percieved as a positive that you will be their coach, use that as well.
Also use a sense of urgency. If they hope to play a HS sport, now is the time to choose that path. If they aren't relatively sharp going into HS, the chances become quite slim.
If the HS program is a successful one, use that. You are giving them a chance to earn their way to be part of something special.
Tell them how much chicks dig guys in a uniform

Build a well-organized winning program that is run with fairness, discipline, respect and encouragement. You will find more and more kids flocking to you.
All this said, I do acknowledge what others are saying. This is the time where you will lose many good athletes to a variety other things. It's OK to make that effort (in fact, IMO, part of your responsibility) but then let it go and focus on the ones who do have the interest in baseball.
Good luck and let us know how it goes.