Learning to write well requires frequent practice. A daily journal entry is a wonderful way to incorporate writing into your homeschool program. Even if you use a writing curriculum, a journal is an easy and enjoyable way to provide the daily practice that most programs lack.
My children look forward to writing in their journals from Monday through Thursday, because that's when we focus on being creative. They write about anything they wish, without worrying about spelling, punctuation, or grammar. They just get their great ideas on paper. They might choose to write a poem, a book report, a movie review, a fictional story, a factual report, or a letter to Grandma. During this initial step, too much concern about the mechanics of writing can distract a child and interfere with creativity.
On Friday, the child chooses his favorite journal entry from the week to critique for language and mechanics. We go over it together, finding misspelled words in the dictionary, correcting grammar, and adding proper punctuation. We look for words that are overused and try out some more interesting options, such as "shouted" or "exclaimed" instead of "said". Dull adjectives are replaced with more exciting descriptive phrases. The children need less help with this process as they gain experience.
When they think their composition sounds just about perfect, they rewrite it on special paper in their best penmanship. These "perfect" essays are collected in a notebook, separate from the main journal. At the end of the year, the student has thirty or so very nice pieces of writing in their portfolio, and it's easy to see how their writing ability has progressed.
We've altered this method at times, specifying a certain type of writing for each day of the week. We've used purchased journals that include writing prompts, paged through magazines for inspiration, and searched for writing prompts online. The subject matter isn't as important as just keeping them writing.