Maintain VFR conditions on top as long as possible to observe and avoid buildups. If getting on top is impossible or becomes impossible to maintain, the best option is to descend as low as possible to what may be VFR conditions below the clouds.
Maintain VFR conditions below the bases to observe obstacles and avoid the rain shafts. Never fly directly below a cell.
Daytime: Don't go where the sky is dark. Talk to ATC and get pireps.
Night: Don't go where the lightning is. Talk to ATC and get pireps.
Choose the best altitude, as low as possible.
Tighten seat belts and secure your flight bag (so it doesn't hit the roof and deposit its contents onto your head).
Pitot heat, prop de-ice and/or engine anti-ice: on.
Cockpit lights: highest intensity. Keep your eyes on the instruments, not on the light show outside.
Slow down. Establish a power setting to maintain maneuvering speed (Va), but do not chase altitude or airspeed excursions.
Extending the landing gear can help to slow and stabilize the airplane.
Tell ATC — calmly and professionally, of course — that you are in extreme turbulence and you are climbing or descending right now. Do not waste time requesting permission. This is a bona fide emergency. Tell them what you are doing. ATC will automatically treat it as an emergency. You are the pilot in command. §91.3(b): "In an in-flight emergency requiring immediate action, the pilot in command may deviate from any rule of this part to the extent required to meet that emergency." It does not say you must first get permission. Do not allow ATC to fly your airplane. You are in charge.
Keep the wings level. It is too late now to turn back. Load factor increases enormously in a turn.
Do not attempt to maintain a specific altitude in severe updrafts or downdrafts. Concentrate on keeping the wings level and the airspeed somewhere in the neighborhood of maneuvering speed.