I've been using my Focus for a couple months now, but I'll be darned, and learned a few things from a recent Windows Team Blog entry on the keyboard.
Science behind the Keyboard
Itais 7 favorite keyboard tips for Windows Phone 7
1. Type fast and trust the keyboard to correct you.
2. Double-tap Space at the end of a sentence to add a period and capitalize the start of the next sentence.
3. Tap & hold various keys to reveal hidden menus. The period key is especially handy.
4. Boost your speed: Tap the Shift or Numbers/Symbols key, slide your finger to the character you want, and then release. Saves you a tap.
5. In the suggestion bar, tap + to teach the phone a new word.
6. To undo auto-correction: tap the word, then tap the first candidate (the original word) in the suggestion bar.
7. Multilingual? Go to Settings>Keyboard to select other keyboard languages.
Science behind the Keyboard