Synchronized SPI communication

I am trying to implement interrupt based SPI communication between two
Atmel microcontrollers. The data has to be transmitted from slave to
master.

In master i continuously sends 0xff to the slave and master expects
data from slave on each transfer.

char SPI_MasterTransfer(char cData)
{
uint8_t a;
PORTB &= ~(_BV(PB4)); // Slave select
a = SPDR;
SPDR = cData;
while(!(SPSR & (1<<SPIF)));
PORTB |= (_BV(PB4));
return a;
}

I have a an array buf[3] = {10,20,30} in the slave. Whenever the
transaction is over the slave gets into the interrupt service routine
and checks if the data received is 0xff, if so then transmits buf
(where i varies as 0,1,2,0,1,2....)

ISR(SPI_STC_vect)
{

uint8_t data;

data = SPDR;

if(data == 0xff){
SPDR = buf;
flag = 2;
i= (i+1)%3;
}
}

Now the problem is i get correct data as well as junk data at the
master side.

What i deciphered the problem is:

After the reception of 0xff from master the slave get into the ISR()
and at this time the master may again send another 0xff for which the
master gets back 0xff (ie., the last data received in the slave SPDR)
itself from slave.

I solved the problem by putting delays between each transfer in master
micro.

But the solution looks a bit (lot) awkward. Can i achieve
synchronization in any other method?? (any kind of talk back
methods...)