This simple procedure will comment multiple lines:

• Go to first or last line of the intended area
• Press CTRL-V for Visual Block
• Go to the last or first line to select the intended area
• Type    :s/^/#    and press enter

Obviously following the same logic we can uncomment a block with:

• Go to first or last line of the intended area
• Press CTRL-V for Visual Block
• Go to the last or first line to select the intended area
• Type    :s/^#/    and press enter

Not an easy one to remember so what do you do? You write it down somewhere, right?