Writing common phrases familiarizes you with everyday words. Make a list of some of the most common things you'd say in daily life (such as "How are you?" and "Where is the gas station?") and translate them into French. Take note of words that are used frequently, like interrogatives and words for common household objects, so you can use those words in other sentences later.
Writing short scripts in French will allow you to consider the perspectives and tenses used in conversation without limiting you to a specific setting. You need only imagine a scenario in your head, and then write how it would work out in French. You can cover all kinds of grammatical issues, like future tense, addressing groups, asking questions or speaking in the negative. Anything you have trouble with is suitable fodder - just come up with a dialogue between the characters in your script, and you can exercise those skills.
Translating existing works into French is good practice for those who aren't creatively inclined. You can translate nearly anything. Novels, songs, movie quotes; even video games can be translated into French for practice. For an extra challenge, translate a song or poem into French, but try to maintain the rhyme and meter of the original. This will make you more familiar with the sounds of the words while simultaneously forcing you to learn alternative ways to describe scenes and express your ideas.
Talking to native speakers is a time-tested method of learning a new language. Short of full immersion, there's nothing that compares to learning a language firsthand from someone who speaks it daily. With that in mind, what better way could there be for practicing your French writing outside of France than exchanging letters with a French pen pal? This is one of the best methods for practice because you'll use practical language skills - you're writing about your life in your own words, and you're getting responses from a real person who can give you insightful tips on the language. Some people may find the idea of a pen pal to be dated or inconvenient, but these days there are a number of websites and services available to match you up with an email pen pal or connect you to foreign-language speakers via instant messenging or chat rooms.