In addition to the great advice given so far:
1. Use your ears!!!! (You NEVER boom without headphones, right??) The earlier comments about the effects of mic-to-source distance and reflective surfaces are dead on. Add to that the level and direction of ambient noise and you have plenty to be aware of. Move that mic around until it sounds good!!! Be searching for the best mic position (for both sound quality and shadows) by having your actors or stand ins talk while your lighting guys are busy setting up the shot. Knowing some sound theory about mic distance and reflection will help you make good mic placement choices faster, but at the end of the day, your ears are the most accurate judge.
2. Be polite, but insistent, when the DP/Director frames and lights the shot with no place to capture good sound. If you have to place your mic 10 feet away from the source to avoid shadows, make it clear to the DP/Director that sound is going to suck. (Of course you must do this with humor and diplomacy rather than whininess and negativity, but you must speak up. It is part of your job.) In the end, the visual may take priority, but the director has then made a conscious decision rather than just discovering bad sound during post.
3. Depending on the level and direction of ambient noise in your environment (air conditioners, traffic, etc.), consider small movements to capture different actors in a two shot. If you flip the mic to completely face actor #1 during his dialog and to completely face actor #2 during her dialog, you will get the hottest signal for each actor, but you may hear the background ambient levels washing back and forth (annoyingly) as you flip. Centering the mic over them and making smaller facing movements may avoid this.
4. Face and body resonances: In general, you will get a lighter, more natural dialog sound by booming from above. Point roughly toward the bridge of the actor's nose as a starting place then adjust by ear from there. Unfortunately, booming from above is also where you often have the most conflict with shadows, so booming from below may be necessary. Unfortunately, you'll often pick up more chest resonance and less of the clarifying high frequencies from below. Just the way the body and face are built. We teach singers to control some of these face and body resonances, for example to switch between a more operatic sound or a more pop sound.
5. The Rode NTG-3. Dang! I swear it sounds as good as an MKH-416, but half the price.