hi,

to involve bootloader you should make next hardware conditions:
PSEN=0, EA=1, ALE=1 or not connected
to involve ONCE^TM mode:
PSEN=1, ALE=0

So it is easy: do pull-up for both ALE and PSEN with external resistors. There is no such combination of signals specified and so no mode should come up but normal start instead.

Some more: in fact, I never did it. Here is my opinion below.
As I understand Atmel hardware, if you need to put a chip into BOOT or ONCE^TM state then you have to use external pull-up/-down resistors or another technique to provide required signals to defined pins. With this you make strong current through defined pins and selected mode is activated. But if you leave pins at float state then no current is produced. And so no mode comes up. Once again: it is my opinion only.

Regards,
Oleg