It's vital that you're honest with yourself in order to find some sort of base cause to any given thought/emotion. I honestly end up catching myself lost in thought after a minute or so more often than not.
It's really a best effort sort of thing. But the more you practice, and the more effort you put forward, the stronger you'll get and the easier it will be to filter the signal from the noise.
When I meditate all I'm trying to do is keep my attention in the present moment; if my mind starts to wander I bring it back, and so on.
I've always loved going on walks, and they've just turned out to be a good way to work through mental/emotional stuff. Walking helps you stay in the moment as well, and when you're focusing your attention on the moment there's always new things to sense.
Let me know if that's not clear enough.
EDIT:
Another important part of meditation is the analysis of your thoughts and emotions. Whatever comes up, try to understand why it did as well as you can.
The past several years I've been trying to find some tangible philosophical ground to stand on. This (desperate) search is and has been the produce of mental illness I've dealt with since my adolescence (I'm 32 now).
Quite a long story short, I managed to get the mental illness under control; something I thought I'd be living with the rest of my life.
My research has included mostly standing/walking meditation, and reading a lot on philosophy, religion, psychology, and such.
This is a personal project I've only just sort of revealed, after some persuasion by my peers. I didn't really have much intention on putting out in the public but it has turned out to be something significant. There is a lot to say about it.
EDIT: If you're curious, here is what I came up with after I started recording my research. DISCLAIMER: there is some personal stuff I talk about.