Preflight Procedures
Systems and instrument checks
IFR preflight is a systems verification, not just a walk-around. Check pitot heat and the static ports, the alternate static source, vacuum or pressure pumps and their gauges, the electrical load with every avionics unit running, and the standby instruments. Set the altimeter to the current reported setting from a station within 100 NM and cross-check it against field elevation; about 75 ft is the usual tolerance before you suspect a static or instrument problem.
Departure planning
A part 91 flight may legally depart in zero-zero conditions unless the operator's rules say otherwise, but you still must be able to fly the obstacle departure procedure. Standard IFR departure planning assumes crossing the departure end of the runway at 35 ft and climbing 200 ft/NM to 400 ft AGL. A published ODP may demand more; if you cannot make the gradient you need better conditions, a different runway, or a visual climb over the airport. Read the takeoff minimums and departure pages before engine start, not after clearance delivery.
Traps
Questions like to confuse departure visibility with approach visibility and to claim charted takeoff minimums bind a part 91 pilot. Watch for "climb in visual conditions" instructions, which authorize a VFR climb to a stated altitude before entering IMC, and for ODPs marked "not authorized" for certain categories.
Rule of thumb: if the destination will be below its approach minimums at your arrival time, plan a takeoff alternate before you taxi.