I think it takes a technical lead who has the vision & skill to balance house-keeping work with new feature development. It can be both technical as well as political skill.

One methodology that I employ is "visibility-based development" where we will save up some super easy tasks that may have a big visual impact. When we need a sprint or two to get some boring clean-up work done, we'll throw in a few of those easy tasks with it. We've had some releases that look like a big, major release but in reality we only spent 1/2 a day on the features that seem big. The rest of the time was under the hood cleanup.

In some cases it's not simply your customers that require the illusion, but the management team as well. If you have a management team that gets uneasy if they don't see new features happening with every release, I highly recommend using this technique.