This has got to be the most useless tip ever. :) Go to the monitor and enter:
>1FFF8 4B
This will make your C128 go into 64 mode automatically every time you reset, and there's nothing you can do about it. ;)
Alternatively, issuing the BASIC commands "BANK 1:POKE 65528,75" will do the same.
WARNING! You will lose your RAM1 content, because you must turn off your computer to remove the reset trap. This works by changing the soft reset vector in bank 1 to point to the GO64 routine, whose execution will take precedence to checking the RUN/STOP key to invoke monitor, for example.