Set your computer to a static IP of 192.168.1.7. (or to whatever subnet the router is on) Disable all firewalls and security. Disable wireless on your computer and only have the router connected to the flashing computer by the ethernet cable between the two.
Hard reset or 30/30/30 (If the router supports it, if not, reset to defults in the GUI) prior to flashing. Wait. Check for password page on re-login and change password.
Flash firmware. You can use the webgui except if you have a belkin router. (For belkin use tftp.exe to flash)
Wait...at least three minutes. Lights should return to normal. See important2, below. Failing to wait is how most people brick their routers.
Do a power cycle of the router. (Unplug the cord, count to 30 and plug it back in.)
Wait for the lights to return to normal usually about 2 minutes.
Hard reset or 30/30/30 again (If the router supports it, if not, reset to defults in the GUI). Wait. Check for the password page and re-login to change the password. Then you can reconfigure your settings manually.
Once configured set your computer back to autoIP and autoDNS.
Important1: This Hard reset or 30/30/30 works fine for Asus router, but you do have to power cycle after the reset.
Important2: After you flash the firmware, and before you do the hard reset, the router will be building some nvram settings. YOU MUST WAIT FOR THIS TO FINISH PRIOR TO DOING ANYTHING WITH THE ROUTER INCLUDING A HARD RESET. Usually, you can tell when this process is completed by the WAN light coming on, but it does take several minutes. Go have a beer. There are starting to be more and more people who BRICK their routers by not waiting until the nvram is rebuilt, PRIOR to doing a hard reset. YOU NEED TO WAIT!
In Windows XP, Click Start Button - All Programs - Accessories -
Communication - HyperTerminal
Enter a name for the connection, Click ok
Choose com port you adapter is plugged into, Click ok
Bits per second = 115200
Data Bits = 8
Parity = none
Stop bits = 1
Flow control = none
Click File - Save As, and select a place to save it to so you
don't have to enter the settings again.
After installing putty, run it
Serial line = The COM port your using for serial (ie. COM3)
Speed = 115200
Click on Serial under Connection
Serial line to connect to = same as above (Serial line)
Speed (baud) = 115200
Data bits = 8
Stop bits = 1
Parity = None
Flow control = None
Enter a name for your connection under saved sessions