Basically, you play various games, most of which are pretty fun - that is, if you're naturally competitive like I am - and only need to spend about 10-15 minutes a day most days of the week to (so they say) see improvement. The areas of focus are flexibility, attention, memory, speed, and problem-solving. The majority of the activities/games that I've done take 1-3 minutes or so.
Brad's been doing the training course pretty regularly. The site tells him what activities to do each day, based on his personal progress, etc., and it's different every day.
I've gone another route - a route that is surely less beneficial, since I'm not getting the varied experience he is, but more fun. That is, I've become obsessed with a game called "Word Bubbles" and its sequel "Word Bubbles Rising," which promise to improve your verbal fluency (whatever that means).
These two games are basically web versions of a game - and this is no joke, despite its excessive dorkiness - that I forced Brad to play over Christmas break. A game I . . . thought I invented. (Yes, I am that absurd.) You get the first so many letters of a word, and you make words that start with those letters. But, it's trickier than just coming up with as many as you can think of - you have to make them a certain length. So, you make three that are 3 words, three that are 4 words, three that are 5, and so on, up to 10+ letters.
Anyway, if you're worried about losing your edge or paranoid about Alzheimer's (as I am, for no reason other than general paranoia), I'd say give Lumosity a try. It's inexpensive, you can do it on your own time, and it's pretty fun. You might even figure out how to spell "veterinarian." It only took me 3 tries.
P.S. I can beat Brad at every single game. Shiz-am!