Ask HN: How do you limit liablity in a company as a minority stake holder?

Ok, so I have a life long friend who is starting his own business and needs some help (website, biz setup, basic accounting, etc) and wants to give me a small percentage of the company in exchange.

Really, I would do it for free, so I'm not worried about getting anything from him financially for my time... if things work out and I see a return so be it.

My concern is, as lets say a 10% owner/share holder, with two other partners what liabilities do I have? I will not be involved in day-to-day operations and at 10% have very little legal input as to what they do as a company. (Although I would server in an advisory roll)

If things were to go south how do I protect myself? I own my own business (which is my livelihood) and don't want any thing that they do affecting what I am able to do with my business. Really I am looking to protect myself from a worst-case scenario.

Any help would be appreciated.