It takes a lot of dedication and vigilance to remain "in the clear," as it were.
It took me about six months of twice-weekly meditation to really quiet down the constant internal barrage of sound in my head; I was feeling really tuned-in and conscious during that period of my life.
I stopped going to my spiritual center for a few months, and the next time I tried meditating, it was like every transmitter in North America had been turned on at once and was being beamed right into my head.
The first time I heard about meditating and letting your mind go blank, or "black" was in the movie, Poltergeist. Jo Beth Williams is trying to console Craig T. Nelson. She's encouraging to let his mind go black like he did back in college, or something like that. I was just a kid. This scene did a great job of illustrating that adults have tons of stresses and thoughts swarming in their heads and it's not easy to think of nothing.
I am pretty good about "going black" and thinking of nothing, especially now. If I do think of something, I can make it wander back out. This worries me in a way. Do I have enough going on up there in that head of mine? Is everything working correctly?
I second walking meditation. That, or even going to meditate in a group and not alone. I still have a lot of trouble going past 5 minutes at home, but I do a lot better when I meditate with a local zen group that alternates seated and walking meditation.
I use visualizations to let the thoughts go. The one that works best for me involves sitting in a train station and watching the trains (thoughts) go by without feeling obligated to get on any of them. If I do get on a train, I simply get off at the next stop and resume my meditation.
It usually takes my mind fifteen or twenty minutes to tone down the yammering, so you might want to try sitting for longer. Meditating at the same time every might help too; your brain sort of gets trained to quiet down at that time.
If there's one book that helped me calm the internal storm and encourage meditation in motion, it's Thich Nhat Hanh's The Miracle of Mindfulness. I recommend really reading, learning, and applying the first couple of chapters, on breathing and awareness.
Congratulations on your meditation. Remember, though, that the goal is not a length of time. The goal is the moment extended.
I love that book, Hugh. I also like John Dado Loori's Bringing the Sacred to Life. When I first started out I found his Finding the Still Point helpful, as well, but it is very much a super-intro brand-new beginners guide.