do you have images in tiff or jpeg format? other image formats take up less space (ie *.png)
If you have featureclasses or shapefiles that are just there to look nice and the attributes aren't important, just delete any fields that you don't need.
Pretty hard to discern what is taking up the most space, but a quick look with Windows Explorer in the folder where you have stored your data would give you a good idea which files and/or folders are the biggest... target those first by either removing unneeded fluff, reducing extents of input files and getting rid of unnecessary attributes are some of the obvious ones. Also check your geodatabases and/or folders, often they have content that you can no longer remember what they are or why they are there... perhaps intermediate processing results? A good house cleaning doesn't take long.
