With this patch you can focus on any of the currently open windows on the current tagset with just one key combination

config.def.h

@@ -109,6 +109,14 @@ static const Key keys[] = {
 	TAGKEYS(                        XK_7,                      6)
 	TAGKEYS(                        XK_8,                      7)
 	TAGKEYS(                        XK_9,                      8)
+	{ MODKEY|ShiftMask,             XK_j,      focusbynum,     {.i = 0} },
+	{ MODKEY|ShiftMask,             XK_k,      focusbynum,     {.i = 1} },
+	{ MODKEY|ShiftMask,             XK_l,      focusbynum,     {.i = 2} },
+	{ MODKEY|ShiftMask,             XK_colon,  focusbynum,     {.i = 3} },
+	{ MODKEY|ShiftMask,             XK_f,      focusbynum,     {.i = 4} },
+	{ MODKEY|ShiftMask,             XK_d,      focusbynum,     {.i = 5} },
+	{ MODKEY|ShiftMask,             XK_s,      focusbynum,     {.i = 6} },
+	{ MODKEY|ShiftMask,             XK_a,      focusbynum,     {.i = 7} },
 	{ MODKEY|ShiftMask,             XK_q,      quit,           {0} },
 	{ MODKEY|ControlMask|ShiftMask, XK_q,      quit,           {1} }, 
 	{ MODKEY|ShiftMask,             XK_e,      exitdwm,       {0} },

dwm.c

@@ -228,6 +228,7 @@ static void tagmon(const Arg *arg);
 static void tile(Monitor *m);
 static void togglebar(const Arg *arg);
 static void togglefloating(const Arg *arg);
+static void focusbynum(const Arg *arg);
 static void toggletag(const Arg *arg);
 static void toggleview(const Arg *arg);
 static void togglewin(const Arg *arg);
@@ -2043,6 +2044,19 @@ togglefloating(const Arg *arg)
 	arrange(selmon);
 }
 
+void
+focusbynum(const Arg *arg)
+{
+	int i;
+	Client *c;
+
+	i = 0;
+	c = nexttiled(selmon->clients);
+
+	for (; c && i < arg->i; c = nexttiled(c->next), i++);
+	focus(c);
+}
+
 void
 toggletag(const Arg *arg)
 {