The easiest way to keep the size of the OST file to a minimum is to manage the synchronization settings of your folders.  You will do well to have a structured folder system in your email with rules that move emails automatically to appropraite folders.  Once in these folders, you can customize the synchronization settings so that you can minimize the size of your OST files by not synchronizing many emails that you do not need offline.

There are many ways to minimize the data that is being synchronized, but I won't go into all the methods.  I will however outline a method to disable synchronization of a folder as that doesn't seem to be very intuitive in the user experience.  Select a folder that you do not want to have synchronized to your local OST file.  Right click and select Properties.  Select the Synchronization tab.  This tab will tell you how many items are on the server and on your local offline cache (your ost file).  Click the "Filter" button and select the "More Choices" tab.  To disable synchronization of all items in a folder, check the "Match case" option and select "less than" 0 in the size section.  This will tell outlook only to synchronize emails that are less than zero bytes.  Compression is good, but not that good, so this will essentially tell outlook not to synchronize any email in this folder.  Keep in mind that when you use this method, you won't see any new emails in this folder, even though the emails will be there on the Exchange server.  You may instead want to synchronize only unread emails so that you can see new emails but not have to worry about old read emails.

The downsite to managing your synchronization settings is that you have to do it manually for every folder.  And you have to do it for every machine you use - the settings do not follow to your additional machines.  Another consideration is that the emails will still exist on the Exchange server even though you do not see them on your workstation.  You may have to adjust the synchronization settings in the future to archive or clean out old emails in busy folders.

Once you delete the emails that you no longer need as well as minimizing the emails that are brought into your OST file through synchronization settings, you can consider compacting your OST file.  This can be done by selecting File | Data File Management... from the menu, Find your exchange connection that has your OST file in the list of data files on the Data Files tab.  Double click the file or select it and click the "Settings..." button.  Switch to the Advanced tab and click the "Offline Folder File Settings..." button.  This will bring up a dialog that has a "Compact Now" button on it.  Click it.  Compacting your offline folder can take some time, especially if the file is large and you have removed a substantial number of emails from your mailbox or made significant changes to your synchronization settings.  I recommend clicking it before you leave work for the evening. 

Alternatively, you can also disable the offline caching mode, then ensure that the OST file is deleted, then re-enable the caching mode.  This will build a new OST file with your current synchronization settings.  If you find that the above compact mechanism is taking too long, you might want to try this approach.