I really like the approach described in Jen McGinn and Ana Ramírez Chang’s RITE+Krug: A Combination of Usability Test Methods for Agile Design. It’s a dense paper, but worth your time. Here’s a key part (my emphasis added):

Prior to using the RITE+Krug combination, the user research process and results had been divorced from the Agile processes, which resulted in the findings coming too late to be acted on. Because of this issue, we integrated the user research with the rest of the process, as illustrated in Table 1.

The team consists of developers, product managers, user experience designers, visual designers, quality assurance engineers, and a user researcher. The user experience designers and product managers work closely with the development team during the feature sprints—answering questions, giving feedback on progress, and fine tuning the feature as it is implemented. The bug fix sprints give the developers time to focus on product stability.

Meanwhile the product managers, user experience designers, visual designers, and the user researcher work on preparing the small set of features that will be implemented in the next iteration (see Table 1). This work includes feature selection, design, user testing, and redesigns. The whole team (including developers) gives feedback on the feature specification and design before it is ready to be implemented. Like others, our design team stays an iteration ahead of the development team. Like Patton recommends, we iterate the UI before it ever reaches development, thereby turning what is traditionally a validation process into a design process.

And here’s the table:

One of the biggest issues with usability testing and Agile is the complaint that testing slows down the process. This seems like a really good way to alleviate those concerns.