While attempting to practice mindfulness - especially within meditation - the idea that we are striving for is best seen in the analogy of stirred up, muddy water (our minds). If we could calm the mind, it will settle and - as with the muddy water - the mud and sediment and random thoughts will eventually settle, leaving clear water (mind).
This is much easier said than done. Most often you'll discover, the mind has a mind of its own. You are not generating these random thoughts, something else is. It is a good habit when you become aware of your mental drifting to have some sort of centering practice to 'bring your wandering mind back'.
I have had a great amount of success with what I call my Paper Bag Technique.
Do you remember your elementary school days when your mother would pack your lunch every day in a brown paper bag? It is these bags that I am referring to; especially the way the square bottom would be folded up to flatten the bag.
I imagine the paper bag slowly being opened and expanded with air. This folded bottom expands and opens first and only then can the upper and rest of the bag open up. It is important that you visualize the bag's bottom downwards (as in the picture).
Have you even watched a sleeping baby breathe?
We tend not to breathe like babies breathe. Babies breathe with their bellies. We, as adults, tend to breathe with our chests.
I think it is because we have been taught and conditioned to do so. A big belly, for a man or woman, is generally frowned upon.
For men a muscular chest is seen as both masculine and a sign of strength.
For women a large chest is seen as being big breasted, curvy, sexy, feminine and fertile.
Whether we have large muscular chests or big breasts or not isn't really important. We can always puff out our chests when we breathe to either fake it or augment it. Expanded chests, tight bellies; that's why we breathe like this.
When meditating we need to breathe like babies; with our bellies.
Breathe in and expand your belly first and then slowly fill your chest. Exhale. Repeat. Simple.
In my mind's eye, I visualize the paper bag with my every breath.
I have found that when attempting to meditate, and when my mind wanders, this practice works well. This is simply a good breathing exercise. We tend to never breath fully or deeply enough.