How do you decide what IT budget line-items to fight for?

In my time as a network admin and manager, there were many occasions during both good and bad times where I had to decide what items needed to be included in my budget and which would not. Some I chose on concrete ROI (product X saves Y dollars because of current expense, time, consolidation, etc.), others I chose on principal knowing even in the absence of concrete ROI, made sense and would help the bottom and/or top-line, some were chosen because I believed that management was not forecasting resource expansion to handle future growth. How do you go about making these decisions? Please post comments below.