IT vendor management: What's your No. 1 tips for avoiding lock-in?

Include a termination clause in the contract and ensure you setup a strong a regular governance framework with the vendor to review their performance. 

Include a termination clause in the contract and ensure there is a strong and regular governance framework to review their performance. There are 3 levels of governance:

Operational meeting, at least on a monthly basis. Here is where you review the day to day activities for your project or product., including SLAs, invoicing, etc.

Strategic meeting on a quarterly basis. This is also known as Quarterly Business Review. Here you should review all products or services delivered by the vendor. You should include a performance scorecard with quantitative and qualitative measures. Also, include a section to review improvement opportunities and innovation

Executive meeting, also known as top-to-top. This on a yearly or bi-yearly basis to review strategic alignment and business objectives.

Ensure good terms to protect you on hand-over / termination of the contract.
Good up-front scenario planning for how you would migrate, and use that to build the contract.
Be careful to retain key knowledge in house, as the saying goes "never outsource the brain".

Quarterly audits and termination in case of noncompliance, should have provisions in the original contract.

Shorter contracts.

My number one tip for avoiding vendor lock in is to research and compare multiple vendors before signing any agreements. Make sure you understand the terms and conditions of the agreement and that you can easily switch to another vendor should the need arise. Look for vendors that offer flexible pricing options or open source solutions that allow you to customize and maintain the solution yourself.