In Ephesians 3:4, Paul says "by reading this you can understand my insight into the mystery of Christ." So if we are going to, by God's grace, undertake to explain the mysteries of Christ, we ought to be clear enough to be understood and vivid enough to open the mind and heart.
Justin Taylor's blog has some writing notes from J.I. Packer and C.S. Lewis. Here's my summary of both:
1. Don't write until you have something to say (Packer). This makes perfect sense. Don't blabber. Make a point.
2. Be direct and clear for your ideal audience (both). Write toward your specific reader. Don't waste words or sentences. Don't use big words when short ones will do. Make your meaning clear. Use the short, crisp word or phrase over the elongated one. Don't confuse the reader but enlighten him.
3. Use shade, color, and light (both). There are places for long sentences and short ones. Try alternating them and mixing them up. Use analogies, metaphors, and descriptive words. Impart color and light with your language. Use both sides of the brain and keep the reader awake. Make him feel what you want him to feel instead of simply telling him.