I like to have a crack at this every now and again because my appreciation of it seems to change as the years go by. This time I'll do it through a series of questions.
How do I know I'm being mindful?
The simplest way to do this is to label your experience as it happens. Keep it casual. Pay attention to what is going on and give it a label - sitting, seeing, thinking, feet, pressure, breathing. If you are able to label what is happening then you know what is happening. This knowing is mindfulness.
Is that it?
That's the beginning. Once you know what is going on you can begin to explore experience and what your mind is doing. This gives you some space around experience and this gap is where the freedom happens. This space separates out the details of your life from what you really are.
Why does it come and go?
The mind is a complex instrument that is rapidly switching between different modes of operation (for want of a better phrase). Mindfulness is not a mission critical thing and requires some effort to maintain (although it gets easier and more continuous over time). You can feel the effort of paying attention and the relative non-effort of allowing the mind to wander off. Here's the rub. When you are being mindful you are shielded from unwise mind states that make life seem a bit shit, but when you drifting off the mind will have the annoying tendency to, well, make life seem a bit shit.
Anyway, I digress. One important thing to note is that part of mindfulness is remembering to be mindful. If you look carefully into experience, the mind has this quick flick every now again to check whether it should be remembering something or whether you are doing what you should be doing - if you insert the intention to be mindfulness into your memory banks then you will pop back into mindfulness as if by magic.
That was a long winded way of saying that remembering is an important aspect of mindfulness.
What are the benefits of mindfulness?
Separating out your experience from who are you is very helpful indeed. This does not, however, stop us from suffering the vagaries of human existence (and this is where a mindfulness course kind of leaves you flapping in the wind - it does not really cure anything). I'm not going to get into what else we need to do to make life a lovely adventure but mindfulness is a useful place to start. Suffice to say that mindfulness offers short term relief to very upsetting things.