Verify the identity, citizenship, credit history and background of anyone with whom you are considering starting a business.  Even if you've known someone your entire life, that doesn't mean you know their legal name, how they handle money, or whether they have any other legal entanglements that could compromise your business plans.  Unfortunately, this firm has seen clients enter business relationships with individuals using false identities and/or fictitious businesses on more than one occasion.

Execute Buy-Sell Agreements to separate personal property from business assets like computer equipment, vehicles and even trademarks and inventions that are being allocated to the company.  You don't want to build a business reliant upon key assets only to discover that your company has no legal right to that property.

Follow corporate governance protocol and administrative procedures.  Establish and comply with sensible operating policies and procedures, hold regular meetings, take minutes, periodically audit company finances, follow sound employment practices and maintain all these records to ensure transparency in operations, minimize liability and keep everyone on the same page.

Be clear about everyone's roles and responsibilities.  No one can read your mind, nor will anyone remember every conversation verbatim.  So make the time to thoroughly discuss plans and expectations, then put it all in writing.

Do your part to make the relationship work.  When you make specific commitments that are essential to the successful operation of the business, follow through.  You should live up to the expectations your partners have in you.  If you are responsible for, e.g., tracking accounting and finances, then stay on top of those matters so your partner who's, e.g., committed to marketing can likewise focus on those responsibilities.

Develop succession planning strategies, unless you want to arrive at your office one day to discover that your new partner is some heartless creditor or the absent-minded widow of your now deceased partner.  You must establish plans now for future worst-case scenarios,   including member exit strategies, retirement buy-outs, and even disability or death.

Hire a professional (like a lawyer) to help you and your partners execute the above steps.  Some matters are much easier for a skilled yet neutral 3rd party to negotiate than the business owners, who may feel compelled to protect their individual interests above those of the company.  So let a professional help set you on the right course for success from day one.

...and if you've neglected any of these steps in the past, be sure to incorporate them into you planning for the new year.