Hotkeys is the name of the game once you start using ZBrush. - Morten Jaeger
A few million polies is fine. I wouldn't work on anything more than 10 million. ZBrush runs on magic. - Henning Sanden
Spacebar: Brush settings
B+S+T (Brush Standard)
B+S+B (Clay Buildup): Best brush in the bunch. Adds/subtracts volume and is aggressive. Easy to screw up your model with.
B+D+S (Dam Standard): A carving tool generally used to add wrinkles or ridges. Z-sub default.
B+T+D (Trim Dynamic): Fantastic for hard surface, rocks, and planes. Makes planes.
B+M+V (Move): Move things.
Hold Shift (Smooth): Regardless of any brush, this will allow you to smooth.
Ctrl+Shift (Trim Brushes): Cuts away what is inside the selected region/brush area.