ANSWER - It seems a very common misconception that meditation is about making the mind as blank as possible, I've no doubt that the average Tibetan monk can pull it off but for most of us such a feat is near to impossible...
What I find works better than trying to blank your mind is to focus intently on one thing to the extent that you block other thoughts from coming through. I often meditate on a colour (usually green) - I hold the colour in my mind and project the image behind my eyes so I am thinking about and visualising green as a single image. By keeping a similar image at the forefront of your mind you may find it easier to minimise distractions.
Meditation doesn't have to be a passive process. I'm usually doing something in my mind when I meditate. Often I mentally walk around my grandmother's old house which has great personal meaning for me. I use this visualisation particularly for centering as it allows me to ease my mind out of the mundane. Focusing on the little details and recalling the textures, the smells and the sounds can be an intense experience and one which helps to transport my mind out of the day to day world to a place of peace and calm.
I'd question the concept that meditation should ever be a blank process. One of the things I do when starting a session of meditation is have a cleansing of the mind ritual where I meditate specifically on all the things worrying me at the given time. I write the worries down as I go and in doing so I am able to clear them from my mind. That form of meditation very much involves focusing on the contents of the mind and wouldn't be half as successful if I tried to make my mind go blank.
I'd also comment that no matter how good or how experienced a person is at meditating there will always be those moments of: "Hey -I'm thinking about stuff when I want to be meditating..." Don't let this agitate you or distract you from your purpose - just play around with visualisations until you find something that works for you. And ironically you may find that adding as much detail into your visualisations as you can will actually work better for you than trying to perform the Herculean feat of emptying your mind.