If you just want to keep up to date on interviewing skills, make a habit out of doing interview-style problems. I don't think you need to worry about wearing a suit or writing on a white board. Careercup is a fairly consistent source of interesting interview-style problems; I find myself perusing it sometimes even when I'm not remotely interested in another job, because a fair number of the problems are interesting in and of themselves and simple enough to not take up much of your time.

There are also plenty of other sites, usually with a competitive edge, like HackerRank or TopCoder. Doing problems like these really is not a waste of time even if you're completely content with your job. While a lot of it may seem like algorithmic trivia, you'll internalize it sooner than you might expect, and then you'll start finding real-world uses for some of these tricks and, more generally, your "mental overload" when thinking through a piece of tricky code will gradually go down and down.

Re: interviewing frequently just for practice, that's great if you're in an area with an abundance of tech companies. I think that could be risky if you're in a city with just a few that you'd want to work at, though. You don't want to bomb an interview at the only other company in your city you'd want to work for just because you were rusty and used them for practice.