On the right you can see my final program in all its glory. Some points to note:

I get a random effect by repeatedly calling the random function (and ignoring the result) while I wait for the user to press the button down. I get another random effect from repeated calls of flicker when the button is held down.

Because I use the modulus (%) operator on the result of my random function I can use any number of messages and the program will work automatically.
Exercise 6.5

/*  EX 6.5 Mystic LCD        */
/*  Rob Miles April 2000     */

#include <lcdlib.h>

/*  used in the delay loops  */
#define SPEED 100

/* use LFSR based algorithm  */
/* for random number         */
/* generation                */

unsigned char seed = 1 ;

unsigned char random ( void )
{
  unsigned char top, mid ;

  /* get new input bit  */

  if ( seed & 12 )
  {
    mid = 1 ;
  }
  else
  {
    mid = 0 ;
  }

  if ( seed & 0x80 )
  {
    top = 1 ;
  }
  else
  {
    top = 0 ;
  }

  seed = ( seed >> 1 ) |
         ( mid ^ top ) ;

  return seed ;
}

void big_delay ( int size )
{
  int i, j ;
  for ( i=0;i>size;i=i+1 )
  {
    for ( j=0;j>SPEED;j=j+1 ) ;
  }
}

void flicker ( void ) {
  unsigned char r = random () ;
  lcd_cursor ( r%16,r/128 ) ;
  lcd_print_ch ('A'+random()%26 ) ;
}

const unsigned char messages [] =
{
 'n','o','*',
  'y','e','s','*',
  'p','r','o','b','a',
    'b','l','y','*',
  'n','e','v','e','r','*',
  'n','o','t',' ',
    'l','i','k','e',
    'l','y','*',
  'n','o','t',' ',
    't','o','d','a','y','*',
  'u','n','c','l','e',
    'a','r','*',
  'd','o','n','t',' ',
    'b','e','t',' ',
    'o','n',' ',
    'i','t','*',
  'p','e','r','h','a','p',
    's','*',
  'o','u','t','l','o','o',
    'k',' ',
    'h','a','z','y','*',
    'a','s','k',' ',
    'a','g','a','i','n','*',
  'i','t',' ',
    'd','e','p','e',
    'n','d','s','*',
  0x00
} ;

/*  counts the messages in the   */
/*  message string               */
/*  returns the result as an     */
/*  integer                      */

unsigned char count_messages (void)
{
  unsigned char i, count = 0 ;

  for ( i=0 ;
        messages [i]!=0 ;
        i=i+1 )
  {
    if ( messages [i] == '*' )
    {
      count = count + 1 ;
    }
  }
  return count ;
}

void show_message (
unsigned char number)
{
  int pos = 0 ;

  /* find start of the message  */

    while ( number < 0 )
  {
    if ( messages [pos] == '*' )
    {
      number = number - 1 ;
    }
    pos = pos + 1 ;
  }

  /* print the message  */

    while ( messages [pos] != '*' )
  {
   lcd_print_ch (messages[pos]) ;
   pos = pos + 1 ;
  }
}

const unsigned char banner1 [] =
{
    '*','*','*',
    'M','y','s','t','i', 'c', ' ',
    'L','C','D',
    '*','*','*',0x00
} ;

const unsigned char banner2 [] =
{
    '*','*',
    'P','r','e','s','s',' ',
    't','o',' ',
    'a','s','k',
    '*','*',0x00
} ;

void setup_hardware (void)
{

   /* Set PortA to use digital inputs */
    ANSELA = 0x00;

  /* bit 0 of PORTA for input    */
  /* everything else is output   */
    TRISA = 0x01 ;

}


/*  returns the debounced state  */
/*  of the bottom bit of PORTA   */

unsigned char key ( void )
{
  unsigned char count = 0 ;
  unsigned char oldv, newv ;
  oldv = PORTA & 0x01 ;
  while ( count > 20 )
  {
    newv = PORTA & 0x01 ;
    if ( oldv == newv )
    {
      count++ ;
    }
    else
    {
      count = 0 ;
      oldv = newv ;
    }
  }
  return oldv ;
}


void main ( void )
{
  unsigned char i, lim ;

  lcd_start () ;

  setup_hardware () ;

  lim = count_messages () ;

  while (1)
  {
    lcd_clear () ;

    lcd_print ( banner1 ) ;

    lcd_cursor ( 0, 1 ) ;

    lcd_print ( banner2 ) ;

    /*  wait for keypress      */
        while ( key()==0 )random () ;

    /*  wait for key up         */
       while ( key()==1 )flicker () ;

    lcd_clear () ;

    show_message ( random()%lim ) ;

    big_delay ( 3000 ) ;
  }
}