Meditation cont'd

I'm really curious about transforming the inner thought life. What am I supposed to be thinking about? Is it sinful to not think about God every second? Well, here's one thing I've just found so far in my search, an excerpt (all emphases mine) from Soul Nourishment First (by George Mueller):

I saw more clearly than ever that the first great and primary business to which I ought to attend every day was, to have my soul happy in the Lord. The first thing to be concerned about was not how much I might serve the Lord, or how I might glorify the Lord; but how I might get my soul into a happy state, and how my inner man might be nourished. For I might seek to set the truth before the unconverted, I might seek to benefit believers, I might seek to relieve the distressed, I might in other ways seek to behave myself as it becomes a child of God in this world; and yet, and yet, not being happy in the Lord, and not being nourished and strengthened in my inner man day by day, all this might not be attended to in a right spirit.

Before this time my practice had been, at least for ten years previously, as an habitual thing, to give myself to prayer, after having dressed myself in the morning. Now, I saw that the most important thing I had to do was to give myself to the reading of the Word of God, and to meditation on it, that thus my heart might be comforted, encouraged, warned, reproved, instructed; and that thus, by means of the Word of God, while meditating on it, my heart might be brought into experiential communion with the Lord.

I began therefore to meditate on the New Testament from the beginning, early in the morning. The first thing I did, after having asked in a few words the Lord’s blessing upon his precious Word, was, to begin to meditate on the Word of God, searching as it were into every verse, to get blessing out of it; not for the sake of the public ministry of the Word, not for the sake of preaching on what I had meditated upon, but for the sake of obtaining food for my own soul.

The result I have found to be almost invariably this, that after a very few minutes my soul has been led to confession, or to thanksgiving, or to intercession, or to supplication; so that, though I did not, as it were, give myself to prayer, but to meditation, yet it turned almost immediately more or less into prayer...

Thus also the Lord is pleased to communicate unto me that which, either very soon after or at a later time, I have found to become food for other believers, though it was not for the sake of the public ministry of the Word that I gave myself to meditation, but for the profit of my own inner man.