Usually the biggest culprits in putting mailboxes over their limits are the Deleted Items and Sent Items folders.  In the picture below, you can see that the Deleted Items folder is 148 MB, which is 74% of the soft quota. In total, the mailbox below is 228 MB, 28 MB over the soft quota.

 

 

 

You can empty folders by right clicking on them and selecting "Empty 'foldername' Folder," where 'foldername' is the name of the folder you are trying to empty.

 

 

 

 

There is an option in Outlook that empties out the Deleted Items folder upon exiting Outlook.  You may or may not want to select this item, but know that it is an option.  You can find it by going to Tools on the menu bar, and then selecting Options.   Then select the Other tab; you can then check the Empty the Deleted Items folder upon exiting.

 

 

 

 

You can also check folder sizes to see which folders are taking up too much space.  Start by clicking on the folder icon on the bottom of the left hand side in Outlook.  Second, make sure that your mail box is selected in the Folder List.  Third, click on the link that says Folder Sizes.  This will pull up another window that you can scroll through and see how big your folders are.