Preflight Check
Make sure all of your batteries are charged or fresh: transmitter, inside the plane, and starter if you're running nitro. Check to make sure all of your servos and servo horns are still securely mounted and in good operating order. Make sure your receiver is fixed in place and that the antenna is still routed correctly. Test the on/off switch. Check the integrity of your landing gear, wheels, skis or floats, and replace any necessary parts. Inspect your engine mounts and muffler. Inspect your prop for damage, especially to the tip. Replace it if you find any. If you're running nitro or gas, check your fuel tank for leaks, making sure the fuel lines are fastened securely. Check all hinges on control surfaces to make sure they're not worn or damaged. Check the integrity of your fuselage to ensure glue joints and covering are still intact. Do this again after your first flight if you're going to go up for a second flight. Label your plane with your contact info. Keep the plane warm for as long as possible inside the car or trunk until ready to fly.
Just before takeoff
Double check all engine controls and control surfaces. Inspect the whole plane, especially wings, for icing. If you're running nitro, run the engine a couple of clicks rich. Run the engine, electric or gas, for at least 10 seconds before takeoff. If your gas or nitro engine is cool, it may be a little harder to start.
In the air
Make sure you're dressed warmly yourself, especially your hands and fingers. A transmitter glove is a worthwhile investment (see Resources). Be very sensitive to how your plane behaves. If it seems heavy or sluggish at all, land immediately. Your plane is likely icing up.
Postflight
If flying gas or nitro, cover the engine with a towel or old oven mitt to keep your engine from cooling too quickly. Wipe all surfaces completely clean and dry, checking inside and out. Remove all batteries and unused fuel. Store in a warm, dry place.