After you've drawn a line, right click on it and choose "Repeat LINE" from menu.
Also, if you added other geometry and want to go back to Line, right click and choose "Recent Input"
From the Help file:
Use the Repeat command to repeat a command. Use Repeat only with commands performed at the command window, not with commands that use a dialog box.
To repeat commands:
Type Repeat at the command prompt.
Type the command to repeat.
The command repeats, prompting you as necessary.
Press Esc to stop repeating the command.