What is default password for linksys WRT54GL routers? How to modify it?

Leave the username blank, and the password is admin. If you set that, it will hold it automatically, unless you use the reset button to set it back to the factory defaults (not something you can do by accident since you have to hold the button in for 5 seconds).

http://belkashop.com.ua/products_files/WRT54GL_V11_UG_EN.pdf