Writing about daily observations can be easier than trying to consciously reflect on the "big picture" issues of life. The implications of life's big tickets often become clearer through the thoughts and observations of daily life, which may not carry the same set of pressures as trying to pen profound reflections. Concrete writing about the events and actions of everyday life will be more interesting for any readers, easier to recall visually for the writer, and can speak volumes that abstract musings cannot.
To tighten up the journal writing, choose a personal experience to write about, approaching it as though writing a short story. First-person perspective is perfectly fine, even typical, for a journal but is certainly not mandatory. The experience can be retold in script form (as though it were a play unfolding), in third person, or in standard first-person recounting. With colorful and concrete language, the experience will come alive and may clarify meaning; this can be especially gratifying to those who struggle to journal factual recounting of events.
When journaling, there is no reason--not even convention--that dictates writing out the details of one's day in prosaic form. Any form of written expression may be acceptable, as long as the effort and motivation meld for meaningful form. An inspired poetic passage can often create more meaningful insight than forced prose, for a writer who is so inclined to write in verse. Since poetry can be quite revealing, it's a viable option for journal writing--but don't force yourself to write poems if you don't want to do so. The idea is to foster consistent and incisive writing.
Journals--especially those done for classwork or written for groups--can often benefit from actual prompts, such as "Write about a fun experience you had" or "Things that I would change if I could." Instead of pigeonholing journal keepers, prompts often expands their horizons for later works by showing writers just what they can do. If you're journaling solo, consider brainstorming for a list of prompts you can answer in your diary. If you're part of a class, suggest prompts or write your own--they can be invaluable tools.