Many things affect AM & FM reception, but not necessarily the same things. AM is also known as 'LF' or low frequency - roughly 1 MHz. At this frequency, radio waves are roughly 300 meters long and a good antenna (depending on design) is 1/4 or 1/2 that long. 50 meter (~160 ft) antennas aren't usually practical except for us Hams so you usually compromise with something shorter. AM stations of interest tend to be local and powerful so you can get away with those little loops and bits of wire. For AM, I'd suggest running 20 feet of wire along your baseboards or up the wall or out the window. Try to stay away from power lines, flourescent lights or other 'transmitters'. Kept low, I wouldn't worry too much about lightning. The long runs of power cable and phone wire in your house will do more damage in even a near-miss.
FM is a different story because it is roughly 88 to 108 MHz with wavelengths close to 3 meters or 10 feet. Here you see traditional 'dipole' or 'folded dipole' antennas that look like 'T's. The main problems here are height & orientation. Get a folded dipole for FM - you can make one with some 300 ohm twinlead or buy one from RatShack. Get a 300- to 73-ohm transformer (little screws on one side with a type-F connector on the other) and hook it to the back of your receiver. Then play with its orientation - North-South, E-W, other variations. You will have to experiment to find the best spot. Lightning will not be a concern.
Hope that helps!
73!
-Brian n8wrl
