Archive, archive, archive.
If you don't want to allow Outlook to do this automatically at regular intervals (default: 14 days, can be set to daily) then create a new PST and use Outlook's manual archiving capability. In 2010, Click File - Cleanup Tools - Archive. Select the 2nd option ("Archive this folder and all subfolders") then select your current personal folders file as the "source"; choose a convenient date for "Archive items older than" and then select your newly created file as the "Archive file" and hit [OK]. Watch the bottom right-hand border of Outlook's window for progress notes - it tells you which folder it is currently engaged in archiving. Note this can take a while! A long, long while, I think, with a 25 GB file.
Once the process is complete, the PST file is still the same size it was before and has to be compacted. It might be a good idea to back it up first though.
Still in Outlook, go to File - Account Settings - Account Settings again - select the Data Files tab. Highlight your main (default) PST file, and click the button for "Open Folder". This opens a File Explorer window in the relevant folder, and you now need to CLOSE OUTLOOK or the next step won't work.
Right-click your large PST file and select "Copy", then right-click anywhere in the white space in the same folder and click "Paste". This will create a copy of the file, named "{same name as original} - Copy.pst". Again, not a lightning-fast process, but depending on your computer and disk drive speed shouldn't take more than a minute or so.
LEAVE OUTLOOK CLOSED. Open your Control Panel and type MAIL. This will show you a CP applet called Mail or Mail (32-bit) - click to open it and select Data Files - you will notice this is the same dialog box you were using just now from within Outlook. Once again highlight your large PST file and click the "Settings" button. Click [Compact Now] and walk away. Go make coffee, watch a movie on TV, have a bath, go to bed, have a good night's sleep, eat breakfast; if you're lucky it will be done. Maybe. Seriously.
I recommend this procedure (doing it via the CP applet rather than from within Outlook) because if OL is open it continues to try to send/receive mails at whatver predetermined interval you have set. At best this is going to slow down the process, but if you are already experiencing issues with send and receive it strikes me it would be best to avoid that while working on the file.
Good luck :-)