You can prevent the Lisa from automatically starting up by not powering on the ProFile hard drive on a Lisa 2/5, or pressing any key other than the CapsLock key after the first click that you hear.
Hold Apple and press one of:
|1||Upper Twiggy drive on Lisa 1, or Widget Hard Drive on 2/10|
|2||Lower Twiggy or 3.5" Floppy Drive|
|3||Built-in parallel port (external ProFile, or internal Widget)|
|4||Slot 1 lower connector|
|5||Slot 1 upper connector|
|6||Slot 2 lower connector|
|7||Slot 2 upper connector|
|8||Slot 3 lower connector|
|9||Slot 3 upper connector|
The upper/lower nomenclature is applicable to the Dual Parallel Port card.
For some expansion card ROMs (eg. QuickBoot, LSAC), upper/lower is a misnomer. In this case, the lower number invokes normal startup, while the higher number may be an alternate (eg. in the case of QuickBoot and LSAC in a machine with two SCSI ports: "use the other SCSI port").
Apple and Enter (on keypad) aborts boot. Apple and Shift P - aborts boot, do power cycling.
Source: Lisa Owner's Guide and ROM source[LisaFAQ] > [Hardware] > [Lisa ROMs] (Comment on this answer)