-Wall unfortunately leaves a lot of warnings disabled. We've used:

"-W -Wall -Wcast-align -Wstrict-prototypes -Wmissing -prototypes -Wpointer-arith -Wshadow -Wsign-compare -Wformat=2 -Wno-format-y2k -Wimplicit -Wmissing-braces -Wnested-externs -Wparentheses -Wtrigraphs"

Update: Forgot the most important one ... -Werror. Making warnings equivalent to errors is the only way to ensure that they're always taken care of.