I have read quite a bit on manifesting lately, and I have to say you've really got to try it out for yourself before making any judgments. It's such an easy thing to dismiss with a wave of the hand without giving it any credit. Simply saying it is luck, coincidence, or a matter of being in the right place at the right time. I think the practice of manifesting and 'putting it out there' in the universe is a healthy practice, whether or not you believe in those "asks" eventuating. Just the simple act of writing it down or shouting it from a cliff-top fills you with a certain buzz that alters how you go on your way.
After doing this for a couple of months now, I have come to the realisation that one way to not only see manifestations eventuating before your eyes, but also add a little spice to your life, is to simply manifest for more surprises in your life. More opportunities and spontaneity. This way you are looking and expecting more of the unknown to pop up before you, and are then are more likely to try it out, and not be so 'oh, no - I couldn't do that' when the opportunity arises. Being a bit of a socially anxious person, surprises of the social variety (aka going somewhere or hanging out with someone at short notice) can often make me with-draw even further into my shell and be so close-minded and definite that the situation will be awkward, un-enjoyable and anxiety-causing. However through asking the universe for such positive experiences in advance, you are giving yourself a heads-up.
This practice has really worked wonders over the last couple of weeks. I will admit and say I am getting better at it slowly, as odd habits and worries often creep back in. But recently, in the form of a surprise email or text, plans have started to form that I am 100% comfortable with from the get-go. It's great to feel that progress within yourself. It proves that you are not stuck in time, constantly battling old concerns, that these can fade and you one day you will be free of them.
Manifesting specifics is all well and good, but there is a point where you can be too set on achieving/receiving/experiencing something specific. So much so that you become addicted to the idea of it, letting it consume your every thought. Thinking about it when you wash the dishes, or drive to work. This takes you out of the present, which is the whole idea of happiness and living a fulfilling life. So as vague and mystical as the whole manifesting concept is already, try and make your "asks" just as generalised and see where that takes you. I think you'll be surprised with the outcome.