When I want to download files from pc to my phone, I have to click several buttons: Settings, Mass storage only, Mount, etc. Because I am developing android app with my phone, it is more inconvenient. So I thought that someone maybe has made an app to solve my problems. I was right. There was an app which is exactly as same as my idea. Its name is Auto Mount Your SD Card. And it is free(Wow).
First, download it at android market.
Then run app by touching its icon. You can make its shortcut on Home screen. Enable SD Card Auto-Mounting.
Now, when you connect phone to pc with usb cable, you can transfer files without touching any button.
However, one thing I don't like about this app is that it doesn't work when I activated "USB debugging". I have to deactivate "USB debugging" and activate "Mass storage only" manually. I will make improved version of this app some day.