Some things i've been thinking about recently in regards to concentration:
The Buddha often uses a fortress on the frontier of a warzone as a metaphor to describe different qualities of mind which make up the path to the end of suffering. Mindfulness is a gatekeeper who decides what to let in and what to let out, and concentration is a supply of food. It happens to be the nature of the (untrained) mind that it can't stand not to feed on things, whether they be sensual pleasures or thoughts of sensual pleasures, or form pleasure or formless pleasure, the mind has to feed until it reaches nibbana. When we say no to our sensual desires, we often find ourselves jumping from thought to thought, compulsively feeding on our sensual fantasies. In such a state we can't be alert to the present moment in a continuous way (as in moment to moment rather than day to day) which is what is necessary to see dependent origination. Concentration can satisfy our minds so that they don't have to slip off away from the present moment into whatever sensual fantasies (or nightmares) they are interested in. If we are paying attention without concentration we can find ourselves slipping off in subtle ways over and over again each second, creating affect and influencing our experience.
So look at sensual thoughts and concentration simply as different forms of 'feeding'. We have to feed a certain amount, so feed in a more satisfying way, which is also independent from other people, harmless, and present, so that we can be satisfied without leaving the present moment for various fantasies and thus miss out on the important decisions we are making here that create suffering. We are automatically extremely alert to our own needs, we really can't avoid this sensitivity, it happens on its own. So it can become a matter of changing our habits to fulfill those needs with changes through the (mental and physical) way we breathe. Any need to feed can be met by this process, and by switching over to this form of feeding we can find the way to unbinding in which we no longer have to feed. The more satisfied this need is, the more present we can be in this moment. By "present" i mean not slipping off into sensual fantasies (in the broadest possible meaning of the phrase) from second to second, such 'slipping away' can be very subtle and when we are outside of concentration, even though we may be trying to be aware of the present moment alone (and even thinking we are successful), the slipping away is more or less constant and we can't get any insight into the four noble truths.
So concentration requires a sensitivity to our needs and a willingness to respond as skillfully as possible to them each moment again, this is why I don't think teaching concentration simply as steady awareness is effective, because it really takes a moment-moment creativity and changing input intention and sensitivity to our changing needs. Each breath (as a mental and physical process) can be slightly different as we try to fulfill this need to feed without leaving the present moment which reverses and changes itself so quickly that even the Buddha admitted he couldn't think of an apt metaphor for it. The best metaphor I can think of is a fish flopping around in bucket out of water, that's kind of how our minds are - constantly, they flop around in different forms of sensuality and can only really settle down and pay real, continuous, effective attention when they are satisfied, and the only way to find a continuous satisfaction in the present moment is through amplifying and modifying the natural refreshment and fullness (through changes in both the physical and mental aspects of breathing) which comes with fulfilling the body's most constant and pressing need - the need for oxygen.
