Below is a short example of the F() progmem macro.
client.println(F("<HTML>"));
client.println(F("<HEAD>"));
client.println(F("<TITLE>Arduino GET test page</TITLE>"));
client.println(F("</HEAD>"));
client.println(F("<BODY>"));
client.println(F("<H1>Zoomkat's simple Arduino button</H1>"));
client.println(F("<a href='/?on''>ON</a>"));
client.println(F("<a href='/?off''>OFF</a>"));
client.println(F("</BODY>"));
client.println(F("</HTML>"));