This is a small yet so useful hack. By displaying the size of files underneath the files itself, you are immediately aware of the files which occupy a lot of space and those which are not. This is especially useful if you are using a paltry 80 GB hard drive laptop like me ;-). Look at the screenshot below and you understand what I mean.
Open Nautilus file browser. In plain English, open your Home folder(for example).
Then Goto Edit - Preferences
Select the tab Display and as you can see in the screenshot below, the first option is None. Change that to Size.
Done. Thats it. Now take a look at your files. Under the files to be precise ;-)