Morning - probably within an hour of awakening.

Your thoughts are usually still running relatively slowly soon after awakening so it is easier to direct them, and thereby also direct the coming day.

If you leave it to later in the day, established thought patterns will probably have set in and you may end up being distracted by the "loudest" (most dominant) thoughts rather than the "best-feeling" thoughts.

The key, of course, is to eventually make your "loudest" thoughts your "best-feeling" thoughts :)

Always morning - start the day well, and you will probably have a wonderful day. It kind of sets the tone for the rest of the day - and as the day goes and you feel good, more good stuff will come to you and reinforce the feeling, which will bring more good stuff and so on.

A lovely video about this subject : http://www.youtube.com/watch?v=Isj2yrd0BF0&feature=related

check your horoscope
the beginning cycle of
your current experience

then map the degrees of
the changes within the day
noting the planetary rule

what impluses favor
the varying aspects of you

Morning of course plus before and while sleeping another perfect time. Here is an illustration from Silva to the levels of sleeping:

State Brain Wave Frequency (cycles per second) Associated with… Beta 14-21 and higher Waking state, the five senses. Perception of time and space. Alpha 7-14 and higher Light sleep, meditation, intuition. No time and space limitation. Theta 4-7 and higher Deeper sleep, meditation. Delta 0-4 and higher Deep sleep. You are unconscious at Delta. During Alpha (Light sleep, meditation, intuition. No time and space limitation.) and Theta (Deeper sleep, meditation.) your conscious and subconscious are in an real obedient state, you can control, direct and own them Scientists call this the Alpha and Theta brainwave frequencies

Regarding creating it is useful too because in these two levels you are not sleeping, you are relaxing your body moves, your mind roaming without interruption, so it is easy to direct your thoughts whatever you want, mediate and imagine before entering Delta level. You can easily form a thought without any resistance to be manifest into flesh.