System tricks:
Kill the Explorer task
This should be the LAST tweak you perform before starting the game.
Windows Explorer is what puts the icons on your desktop, and the taskbar on your screen. These take up a fair chunk of resources, which do absolutely nothing while you're playing. So kill it.
If you're running Windows XP, you do this by opening the X³ launcher window. Then hit CTRL ALT DEL together. Go to the PROCESSES tab of the Task Manager, and find the Explorer process. Click END TASK and when asked, click YES. Icons and taskbar promptly dissapear, so close the Task Manager and now you can play!
When you quit the game, you won't have any icons, a start button or a taskbar. Hit CTRL ALT DEL together again, and then click FILE. Choose NEW TASK (RUN)... and then type explorer into the runbox followed by hitting enter.
Now you've got your desktop back.
Expected frame rate increase: Noticeable to significant.
Turn off all non-essential programs!
You do not want your antivirus running while you play. Nor your firewall. MSN and other chat mediums you definately don't need while you're in X³. Anything in the Taskbar? Close it, until all you have left is the volume.
MAKE SURE YOU HAVE DISCONNECTED FROM THE INTERNET BEFORE DISABLING FIREWALL(S) AND/OR ANTI-VIRUS!
Expected frame rate increase: Depends how many programs you had open.
Defragment!
A heavily fragmented drive is a slower drive, because the read heads have to chase the data from all over the place, jumping little holes created when you deleted that JPG of Angelina Jolie, or any other file.
Expected frame rate increase: Nil to Noticeable, depending on how bad your drive was.
Yes, it's boring, it takes time, but it's worth doing regularly.
Processes
Use task manager, (or on 98se, a 3rd party app such as Sysinternals Freeware Process Explorer to identify and kill processes you don't need.
Virus scan/spyware scan!
Spyware, Malware and virii can cause havoc to your computer, in some cases completely trashing your data. However, even the innoquous ones have negative side effects, most importantly here on your computer's resources. Less resources = lower FPS, so update your AV and spyware software, and scan regularly!
Expected framerate increase: If you've been bogged down with loads of virii and spyware, the results could be phenomenal!
Ingame tricks
Turn off the HUD!
If you're not using it, turn it off! Heck, try and learn to fight without it! Not only do you get a better view of the sector, your ship and passing traffic, but you get a whopping FPS increase!
Expected frame rate increase: Significant! Some users have reported it's more than doubled their FPS! *this has been significantly reduced since 1.2.01, because the hud doesn't cause as much of a drain*