This trick is mostly intended for lazy people, like me :tongue.gif:
It allows you to start Minecraft without having to log in every time, and it even allows you to join a server directly after clicking the Minecraft icon, if you want to.
If you have the Minecraft.exe on your desktop, you'll have to move it. I put it in C:\ but you can put it anywhere you like.
Now make a short cut to Minecraft.exe, and place it on your desktop.
Now, right-click the short cut, and click properties. If you're not in the "Short cut" tab, click it. Now, in the upper text box (the one with Minecraft.exe in it), add a space, your username, and then your password. If you also want to auto-join a server, add another space and the IP of the server.
Note that other people who have access to your desktop can easily retreive your password this way. So if you have brothers/sisters using the same user account (not Minecraft account), be warned.
Here's a little screenshot of what it should look like:
Thanks to Nekoyasha for providing the image.
If you run Minecraft from commandline, it should work as well. Just add the username and password after Minecraft.exe in the command to start Minecraft. This example is for Linux, but the Windows and Mac commands will be mostly similar. Only the path to the Minecraft.jar may be different.