I would say that you should not be attached to the thoughts, rather than observing them as they go. If you consciously observe your thoughts, you would then be almost repeating the same process that your mind goes through in generating them in the first place!
You don't want to be in the position of feeling like a man in the back row of a cinema watching a film and thinking about what passes.
Just let the thoughts happen and let them go. Don't try to stop themcoming in the first place and don't aim for a blank mind!
One good practice I got from a Tai Chi Teacher was to do with sound. Just sit and each time you hear a sound, rather than think about the sound itself just mentally say 'sound'. The idea of this practice is that you gradually learn to hear a noise without attaching any thoughts or meaning to it.
I find this to be effective.
remember that this is just my -current- view : )