What specific mechanisms or processes can help maintain alignment between IT strategy and business outcomes?

If you're a CIO you are focused first on the business outcomes.  The company's business objectives are your objects.  IT is your toolbox for achieving those objectives.  Your IT strategy is the expression of how you are utilizing technology to achieve business objectives.  That simple understanding is the foundation for ensuring alignment. The specific mechanisms or processes are all for achieving that purpose - Demand Management systems, ERPs, LMSs, PLMs, SCMs, CRMs, BI...etc. should all be measured and calibrated to that using project charters, project and program management evaluations, KPIs, and Metrics

The one I have found most useful is to create a Project Charter for every request. Always asking the business to think about how the request/project will impact or change their own metrics. then build a real ROI for any project. I have found most of their requirements do not impact the results and that also helps manage the priorities.
In the case of internal IT strategies, you need to think which "big Y" is being impacted in the company. 

Good Demand Management enables you to understand where the business needs to go.

I’m always saying “Follow the money”. How much of the discretionary IT spending does the business control? Only 0.01% probably low, 30%, better, 70% best

However you need guard rails on continuing maintenance costs of that discretionary spending. Otherwise your ongoing expenses will spiral out of control