These are my favourite gcc warning flags.  The -O is required to
activate the -Wuninitialized, but the -O of course makes debugging
a bit more inconvenient.

-Wall -O -W -Waggregate-return -Wbad-function-cast -Wcast-align -Wcast-qual -Wconversion -Wc++-compat -Wdisabled-optimization -Wendif-labels -Wformat=2 -Wlarger-than-256 -Wmissing-declarations -Wmissing-field-initializers -Wmissing-format-attribute -Wmissing-noreturn -Wmissing-prototypes -Wmissing-noreturn -Wnested-externs -Wpointer-arith -Wredundant-decls -Wshadow -Wshorten-64-to-32 -Wsign-compare -Wstrict-prototypes -Wwrite-strings
-Wdeclaration-after-statement (gcc 3.4+)
-ansi -pedantic for extra pain (but does not work in Solaris)
-std=c89 for extra strictness (but does not work in e.g. HP-UX)
-Wtraditional portability differences with pre-ANSI compilers
-Wfloat-equal sometimes (when feeling extra cranky)