Like meditation or prayer, gratitude may, in some cases, be considered a spiritual practice that one needs to touch base with regularly in order to stay connected at the center.
The practice of gratitude, as all else, cannot be an empty practice, but must be one that connects with feeling. However, as with meditation, it can be a process and experience that one gets better at with attention and in-tention, so that even if one starts out in the beginning with not very much gratitude at all, the prayerful desire to open the heart can begin to have its effect.
As with all other things, one must look clearly and closey at whatever obstacles may be in the way of feeling grateful. Ordinarily, things like judgment, anger, depression, and weariness or fatigue can prevent one from feeling grateful. In more unusual circumstances, encounters with intensely dark energies can cover gratitude so that it feels totally unreachable. The inner emotional attitudes can be dispelled through the ongoing practice of purification and alignment with God. The covering by other energies often requires a period of waiting and patience before the light of one's own soul can be experienced.
For one starting out in this direction, it may seem more convenient to summarize one's grateful thoughts at the end of each day before going to bed. However, this tends to formalize things and, for some, may not touch the individual moments of each day. Another way of looking at the practice of gratitude is simply seeing everything and especially every one during a day as 'gift'. This would include the not-so-pleasant interactions as well as the pleasant ones.
Seeing life as a gift and God as the Source of that gift in all its forms is the essence of gratitude - a state of being that can become as natural as breathing.