SMALL PRINT!: Please ensure you make a back up copy of any files before amending them.  Mutley's Hangar advise if you are not confident in making any changes then please do not try them.  We will not be held responsible for any problems encountered.

Display low, average and high frame rates:
Add the following line to the end of the [MAIN] section of the fs9.cfg file.
Ave_Frame_Rate_Display=1
Once you have saved your configuration file and started your flight simulator press Ctrl+Z twice. You should see the new low, average and high frame rate values

Real 'Real World' weather:
My tip is to visit Met Office Aviation Then you can download the REAL real weather on the proper Form 214 and Form 215, (you can register for free) and enjoy decoding TAFs and METARS for your area. They look incomprehensible, but the key to the code is on the site under "help". 
Using the "advanced" tab on the weather menu, you can enter the real winds at ground level (from the METARS) and winds aloft from the Form 215 into MSFS. If you make these your "global" settings, you will then have stable "real weather" to fly through, over the whole of your local area. You can do your flight planning from the real-world information, just as you would if you were flying for real. Then, when you get lost, you can rightly blame your rotten navigation! (Thanks to John Read for this one)

http://earth.google.com:
Google Earth has a flight simulator mode! If you press Ctrl G while looking at an area in Google Earth you will put the program in flightsim mode. You can tell that you are in flightsim mode when the cursor changes from a hand to an airplane. Once it is in flightsim mode, you can control your "aircraft" pitch and bank by holding down the left mouse button, and moving the cursor around the screen. This will also control altitude if you are moving horizontally. To control speed (horizontal motion), hold down the right mouse button and move the mouse forward for faster, and backward for slower. You can reduce your speed to zero (hover) and can even fly backwards!  To return the program to its regular "trackball" mode, simply press Ctrl T. It takes a lot of practice to get used to the controls, but it is incredibly fun to actually be able to fly around an area that tooks just like it does in real life. I have just spent the last several hours flying around the area that I live in and doing touch and goes at local airports.

Removing all screen messages for screenshots or movies:
Add the following line to the end of the [MAIN] section of the fs9.cfg or FSX.cfg file.
HideInfoText=1
This will remove messages such as replay, brakes, virtual cockpit and zoom etc. and takes a little getting used to!

FSX Tips, Tricks, and Tweaks Guide:
To view Greg Germanowski's excellent guide, click here.

Tips from Concorde Guy:
 

FS9 and FSX: Low frame rates?
Experiencing low performance in FS9 and FSX? Look no further!

Hardware:
First of all you need to consider looking at your hardware. Most components aren’t up to the job for FS, so maybe a new PC or an upgrade is in order. My computer specs are an AMD Athlon 64 3800+, BFG 7300GS Nvidia 256MB and 1GB RAM, and I get 23 FPS (frames per second) with most sliders to the right.

If you have lower spec hardware than me or have better spec hardware and you are getting low performance, then something needs to be done about this! (If you want to find out your FPS, simply press Shift key +Z +Z whilst playing.)

Computer health:
Have you done a spyware scan or a virus scan? Maybe a nasty Trojan has infected a FS texture…this can lead to performance decrease.

Do you run a disk defragmenter? Defragmenting a disk will move the files around so the disk can be more efficient. A hard disk stores data in a physical location, with adding and deleting files the drive will store fragments of data to fill in the gaps. Over time files become highly fragmented and it takes the drive longer to retrieve the data. De-fragmenting will allow faster data read times.

You will find default defragmenter by going to Start - All Programs - Accessories - System Tools - Disk Defragmenter. The default windows defrag is fine but if you want to do reclaim more fragmented space then get a programme called “Diskeeper” which is payware, but well worth it.

Junk:
Freeing up space on your disk is a next step for performance. Delete files you no longer need and uninstall programmes that are useless. Then run a neat little programme called “disk cleanup”, which is located at Start, all programmes, accessories, and system tools.

Drivers:
The latest Graphics card drivers and sound drivers are vital. Latest drivers increase the looks on graphics and performance, not to mention bugs.

If you have a Nvidia card, then see here: http://www.nvidia.com/content/drivers/drivers.asp
For ATI users, look here: http://ati.de/support/driver.htm

Sound cards, (Creative, etc), visit the manufacturers website for those with the model you have.
For on board sound, go to the manufacture’s website of your motherboard (eg Asus, Soltek)

FS config files:
A quick edit of the config file (FS9.cfg or FSX.cfg)  ( See disclaimer above)

You will find the config file in My Computer/Local Disk (:C)/Documents and Settings/{your username}/Application Data/ Microsoft/FS9 (or FSX). Open this up using Notepad.

This file holds an important set of data for FS. It is highly recommended that you back this up before editing. If you forget and you mess up, you can simply delete this, and a new one can be built up by playing FS, although your saved settings will be erased.

In the [DISPLAY] section there is this line:
TEXTURE_BANDWIDTH_MULT=40 - change this to 400 (FSX should have a value of 30, change that to 90) This will increase screen rate immediately

FSX settings:
You have top spec hardware, and you only gain 5-10 FPS? Don’t worry…it’s the settings!
Open FSX, then click settings, then customize.

Hardware section:
Raise the setting for Frames Per Second target slightly to about 20FPS.
Global texture: Resolution to “Ultra high”. 
Resolution: Set at 1024x768@32 bit.
Anti-aliasing off (Have this enabled in your graphics card settings)
Filtering: Trilinear.
Light bloom: Off

Scenery:
Level of Detail radius: Large
Mesh complexity: 75
Mesh resolution: 38M
Texture resolution: 1M
Water effects: 2X low
Land detail: on
Scenery complexity: Extremely dense
Autogen density: Sparse
Ground scenery shadows: Off
Special affects detail: Med

Traffic:
ALL OFF!

Weather:
Thermal visualization: None

Still low FPS? Adjust to your tastes.

If you are getting a new PC, a suggested upgrade is:
Intel core 2 duo E6600, E6700 or extreme.
Asus Motherboard
Nvidia 8800 GTX graphics card
2GB RAM