You spend a lot of time with your friends. You eat meals with them. You might even live with them. Why shouldn't you work with them, too?

Mention this to anyone you know, and they'll shake their heads. Mixing your business life and your personal life could end in disaster, they'll say. That will spell the end for the business and for the friendship.

But not necessarily -- what if your friend also happens to be a perfect fit for the business? What if he or she can offer you something (a service or a product) that no one else can? Are you supposed to just ignore it because you were friends first?

The truth is that plenty of the world's most successful entrepreneurs started successful businesses with their friends -- just look at Facebook. If you learn how to work together, and to respect each other and your newly complicated friendship while putting business first when it needs to come first, you can mix friendship and business.

Here are five ways to mix friendship and business.

First, make sure you can truly trust your friend

Yes, you have a great time together and share a ton of laughs. You have mutual friends and common interests. But that's not necessarily enough to make you good business partners. Make sure that you know how your friend reacts to certain situations, what her beliefs are, his temperament, and how he reacts to stress.

How does she handle tough times and high-pressure events? You need to know these things before going into business with someone.

Does your friend have a skill that you can utilize?

It's a potential win-win situation. Your friend has a talent that you just happen to need. You can give her work and a steady paycheck. You'll get to spend more time together to boot.

Before you envision going for lattes on your break and jump into working together, sit down and discuss what you both need and want from this working arrangement, and sign a contract. It could save your friendship if things go south.

Establish ground rules

If you plan to enter a partnership with a friend, be sure to set some ground rules. What is friend time and what is work time? Who is investing what money, time and space? How will you divide the workload? Be sure to create rules you can both live with, and get all of this in writing.

Set aside time to just be friends

If you're starting a business together, it can be very time-consuming getting it off the ground. You might found that you spend all of your time talking about the business, hashing out ideas, arguing about money, and that all the things that made you friends in the first place have fallen by the wayside. Make time to grab a drink together or see a movie once in a while.

Know where to draw the line between friendship and work

Remember that when you're working together, it's just that - you're at work. Save friendly banter and familiar chatter for when you're away from the office, or your other workers could claim favoritism, or worse yet, leave their own professional behavior at the door.

I asked Stephen T. Johnson, the CEO of FlipMass for his opinion, "There is a fine line between the people you used to go out with, and the people that you're building something bigger than yourself with. Choose wisely when it comes to mixing business and pleasure."

Mixing friendship and business can be done, as long as you plan, discuss the details, remain professional at all times and mind your behavior in the workplace.