Setting up a Minecraft launcher on Ubuntu 12.04, these steps worked for me, mostly pieced together from the various other tips.
In a terminal, first fetch a Minecraft image icon:
cd /usr/share/icons; sudo wget http://images.wikia.com/yogbox/images/d/dd/Minecraft_Block.svg
Then create a minecraft.desktop file (using the gedit text editor to keep it user-friendly; just enter the whole "gksudo ..." command at the command line):
gksudo gedit /usr/share/applications/minecraft.desktop
and paste in these contents, and then Save of course:
Finally (almost), open up your file manager and go to /usr/share/applications - either click the Home Folder icon in your launcher and navigate, or here's the terminal quickie:
nautilus /usr/share/applications &
Locate your new Minecraft entry, and drag it over to your launcher bar.
At this point, you're practically done, except in this case I've created a little Minecraft starter script, saved in /opt/minecraft.sh, which assumes the minecraft.jar file is also saved in /opt. You can change all these "/opt" references to be wherever you've saved the original minecraft.jar file (be sure to change the Exec value in the minecraft.desktop file too).
The minecraft.sh file needs to contain just these three lines:
cd $(dirname "$0")
java -Xmx1024M -Xms512M -cp minecraft.jar net.minecraft.LauncherFrame
and of course that script needs to be executable, e.g. with the command
sudo chmod a+x /opt/minecraft.sh
That's it! Using the starter script, and ensuring it cd's to the folder that contains the minecraft.jar file, seems to be a helpful step that other instructions overlooked.