Dear Usability People,
I am a HiPPO. I have a website that I'm really proud of because it looks awesome and people tell me it's great. Unfortunately my website isn't selling as many things as I would like and so I am emailing you to request your consultancy.
At this stage I would like to point out that I do not consider any of my past, current or prospective clients to be a Hippo! I just found the above image too tempting not to use.
So down to the business of why I'm rambling about HiPPOs and KISSing them:
HiPPO refers to the 'Highest Paid Persons Opinion', a regular aid or stumbling block in any usability project, depending on your approach. KISS refers to the KISS Principle, outlined below;
The principle most likely finds its origins in similar concepts, such as Occam's razor, and Albert Einstein's maxim that "everything should be made as simple as possible, but no simpler". Leonardo Da Vinci's "Simplicity is the ultimate sophistication", or Antoine de Saint Exupéry's "It seems that perfection is reached not when there is nothing left to add, but when there is nothing left to take away".
What Albert and co are trying to say is sometimes less is more.
This is easily applied to usability itself as it's important not to over complicate processes, from requesting more information than is necessary in a checkout process to clouding calls to action with superfluous content. With experience, 'Keep it Simple Stupid' becomes second nature when looking at a website, but HiPPOs are a bit trickier.
HiPPOs are ever changing, they aren't based on facts and as the name describes, rely heavily on opinion. This makes them particularly dangerous to a project (granted, a real Hippo would probably be even more dangerous), as changing a HiPPO can be extremely difficult. That is why it is essential to KISS them. If you can keep your approach, your reasoning behind the approach and the way in which you describe both of these as simple as possible, then your message should be clear and influential.
The only way to change a HiPPO is with reason and the only way to provide a clear reason is to KISS.