Your job is to make sure the muse knows where you're going to be every day from nine 'til noon or seven 'til three. (Stephen King)
If you're going to succeed as a writer, you will have to abandon, if you haven't already, the idea that you need to be properly "inspired" before you sit down to write. There are two basic reasons for this. First, waiting for inspiration undermines the stability of your writing process. Second, it implies too close a relationship between the surface of your style and the depth of your ideas. I'll try to say a bit about each of these.
The muse is famously independent—it is, indeed, often outright undependable. You never know when inspiration is going to "strike" and you never when your muse will get bored and leave you in mid-sentence. Making inspiration a pre-condition for writing, then, can be an enormous inconvenience. Moreover, if you live your life at the whim of your muse, he/she/it might have a difficult time finding you. As King points out, a regular writing process is a way of making sure your muse knows where you are, it makes a target of your mind so that inspiration will have an easier time of hitting you.
The second worry I have about depending too heavily on inspiration is that it undermines the authority of our uninspired self as a critic. If you write only in fits of inspiration you may find yourself estranged from your own writing, unable to assess its value, and unable to improve it through editing. You will write in bursts of brilliance and then send your results off to others on the assumption that they will recognize your genius. But your "genius" will in fact be someone other than you. Writing will become a species of possession (whether angelic or demonic)—you will not, at bottom, know what you are doing, and how you are doing it. "It made sense at the time" is not a good thing to have to say about your own text.
The best way to break your dependence on inspiration is to learn how to use a notebook. Carry a notebook with you in which to write down ideas that "come to" you. And never let a moment of inspiration cause you to sit down to write prose as such. Just jot the idea down, reflect for a moment, and then keep doing whatever you were doing. Just make sure that part of your writing schedule includes time to convert your notes into prose, i.e., time to relate your flashes of inspiration to your ongoing writing projects in a calm, reasoned way. If your muse feels like you're just writing his/her/its input down in the notebook in order to forget it, he/she/it may get more aggressive. Don't say you haven't been warned.