I've been just swimming at the same speed for as long as I have time/energy. Which is kind of dumb really. I wouldn't do the exact same distance and speed every day running so why do it swimming? I always tell new runners to follow a training plan and I wasn't even following my own advice.
So I got a book from the library, "Fitness Swimming", which has a bunch of great swim workout plans and incorporates some crazy drills to enhance my form and speed. This was Day 1 and it went well - definitely made it less boring.