Short answer is you don’t.
You should archive off some data to another PST file. The JET database engine (which is what the PST files are based off of) has a habit of having its files become corrupted once they get bigger than one gig in size, especially when written to regularly.
Personally I break by PST files down by year. This usually keeps them from getting larger than 1 Gig in size.
In Outlook 2003 and 2007 there is a new PST format that offers more capacity, and is capable of storing unicode data. The size limit for this type of file is theoretically 20 GB.
This format is not compatible with PST files from previous versions of Outlook, and I think you cannot “convert” one file from the previous format to the newer, but I guess you can create a new PST file, and copy and paste mails from your old file, or you can import mails from the old file using the “Import and export” option from the “file” menu.
Have a look a the following article. You might find it interesting.
How to configure the size limit for both (.pst) and (.ost) files in Outlook 2007 and in Outlook 2003