"Short-term missions is about partnering, not helping. We now have the privilege of coming alongside the people who God has called to love for the long haul. When we see our role as that of partner, an exchange of blessings occurs. We become givers and receivers rather then saviors. For this to be successful, we must find organizations that not only are established and committed to that particular community, but also organizations we can trust. The more you trust an organization, the more you truly can partner and celebrate all that God has done before you got there, is doing while you are there and will continue to do when you leave." (taken from an article on youthworker.com found here.)
I completely agree!! Having grown up with short-term missions in my youth group and now as a full-time missionary Short-term trips are amazing and can be very impactful. I think some of the downfall is when you don't partner with local missionaries or organizations because there isn't that built in follow-up.
Another thing to be aware of is that the local missionaries and organizations know the needs and culture of the place you are going so it is best to work WITH them instead of insisting on doing things your way because it may hinder instead of help.
For first time trips, partner with a missions organization that does this (such as YWAM, Mission Adventures or AIM). These types of organizations set up trips for groups like you all the time and have for years (some as long as 50) and have access to locations around the world!! They are on the ground and will remain after you are done. There is also the opportunity to build relationship with those missionaries and continue pouring into their ministries long after you have left.