diff --git a/Makefile b/Makefile
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 
-CFLAGS = -Wall -g -O2
+CFLAGS = -Wall -Wextra -Wno-unused-parameter -g -O2
 
 headers = $(wildcard src/*.h)
 sources = $(wildcard src/*.c)
diff --git a/src/vcprompt.c b/src/vcprompt.c
--- a/src/vcprompt.c
+++ b/src/vcprompt.c
@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -13,10 +14,12 @@
 #include "bzr.h"
 */
 
+#define DEFAULT_FORMAT "[%n:%b%m%u] "
+
 void parse_args(int argc, char** argv, options_t* options)
 {
     int opt;
-    while ((opt = getopt(argc, argv, "f:d")) != -1) {
+    while ((opt = getopt(argc, argv, "hf:d")) != -1) {
         switch (opt) {
             case 'f':
                 options->format = optarg;
@@ -24,6 +27,22 @@
             case 'd':
                 options->debug = 1;
                 break;
+            case 'h':
+            default:
+                printf("usage: %s [-h] [-d] [-f FORMAT]\n", argv[0]);
+                printf("FORMAT (default=\"%s\") may contain:\n%s",
+                DEFAULT_FORMAT,
+                " %b  show branch\n"
+                " %r  show revision\n"
+                " %u  show unknown\n"
+                " %m  show modified\n"
+                " %n  show VC name\n"
+                " %%  show '%'\n"
+                );
+                printf("Environment Variables:\n"
+                " VCPROMPT_FORMAT\n"
+                );
+                exit(1);
         }
     }
 }
@@ -37,7 +56,8 @@
     options->show_modified = 0;
 
     char* format = options->format;
-    for (i = 0; i < strlen(format); i++) {
+    int len = strlen(format);
+    for (i = 0; i < len; i++) {
         if (format[i] == '%') {
             i++;
             switch (format[i]) {
@@ -72,8 +92,9 @@
 {
     int i;
     char* format = options->format;
+    int len = strlen(format);
 
-    for (i = 0; i < strlen(format); i++) {
+    for (i = 0; i < len; i++) {
         if (format[i] == '%') {
             i++;
             switch (format[i]) {
@@ -149,11 +170,16 @@
 
 int main(int argc, char** argv)
 {
-    options_t options = { 0,            /* debug */
-                          "[%n:%b%m%u] ",  /* format string */
-                          0,            /* show branch */
-                          0,            /* show unknown */
-                          0,            /* show local changes */
+    char* format = getenv("VCPROMPT_FORMAT");
+    if (format == NULL)
+        format = DEFAULT_FORMAT;
+    options_t options = {
+        .debug         = 0,
+        .format        = format,
+        .show_branch   = 0,
+        .show_revision = 0,
+        .show_unknown  = 0,
+        .show_modified = 0,
     };
 
     parse_args(argc, argv, &options);