I've always planned and managed to take two to four months per year off: between jobs, or simply blocking my freelance-agenda for a few (slower) months. During these months, I take on a personal project with a certain focus.

I don't force myself to release anything: though shipping is nice. I don't set any hard goal.

Just two, three or even four months of playing around and learning. A new Database-concept. Geocoding. TDD. A new framework or language.

This is not enough to become expert or even good at anything. But it allows me to keep up with the field and ever evolving technologies; I'm a webdeveloper, so my field is rather fast-moving.

And, because I plan this ahead, I can afford the time off (by asking a little more during the rest of the year) and I can actually tell clients "nope, I don't have time the coming X weeks".