How to place limits on IT demand

I do not think my IT Branch is any different from any other in the world. .Demand for services is increasing incrementally on a daily basis – yet the resources required to support this growth (people and funding) does not match this growth.
Management attitudes of ‘we also need this?’, without determining the impact properly means that it is always a battle to meet demand at the expense of other IT issues.
Whilst the situation is manageable, I am looking for suggestions to help improve, or manage ‘pressures’ to do more with less.
What mangement tools can I use , not to stopprogress or requests, but to ensure that in accepting, a 2 way transaction occurs.
Your thoughts, suggestions and experience is more than welcome.