What does it take to build a strong youth ministry? Some say it's all about the program: vibrant worship, relevant preaching, cool giveaways, epic games and so forth. The content of your program does matter of course, but it is not the be all and end all. I would suggest that one component is extremely important: Relationship.
Let's put it this way: imagine you were invited to a party in which you had to put up with bad food, poor choice of music and maybe some lame attempts at getting a game going. All these things you will forget and forgive eventually when the party fades into memory, but one thing that might stick longer is: if nobody talked to you at the party.
The horror! Imagine yourself standing in a corner trying to look like you are meant to be there or getting introduced to a bunch of people who do not really seem keen to hang out with you... how awkward! I'm not sure if we realised this, but this often happens to a lot of people who come through our programs and churches.
Having a cranking program but no follow up can be counterproductive, and the most effective way of making someone come back is to make them feel welcomed. This to be honest, takes a lot of intentionality. When I first took on the youth ministry I am in, the one thing I had to do was drill into my leaders that they needed to break out of their cliques and to start looking out for people.
So apart from just focusing on getting the worship, the games and the preaching right; make sure you (and your leaders) put extra effort into befriending and connecting with both existing and new people.
One problem that many ministries face is that they can become so comfortable and familiar with the usual regulars that newcomers find it hard to fit in. This too takes intentionality to overcome (I actually forbid my leaders to hang out with their friends during youth, they can do that on their own time).
We have to remember that people matter more than the programs, and discipleship is most effective when done via relationships. Build a culture in your ministry where people are cherished and celebrated; where everybody who walks through the door feel like they belong - then your ministry will definitely grow!