From the home screen of the Kindle press the "del" button to show a search/filter bar. Type ;debugon and click enter/search and you can type the following shortcuts:
~disableindexing ~disablescreensaver ~dumpindexstats ~exec ~help ~indexstatus ~meminfo ~reloadcontentroster ~resumescreensaver ~startindexing ~stopindexing ~usbNetwork
To enable ssh on the kindle you must type ;debugon and then ~usbNetwork.
When plugging the Kindle into a computer it will go into disk mode, disabling SSH. When plugged into a Pi you may need to actually install the eject command as umount won't take it out of that mode (in fact it may not even have automounted anyway).
Along the lines of:
eject /dev/sda1
sudo udisks --eject /dev/sda
As mentioned above there is a ~disablescreensaver and ~resumescreensaver shortcuts but apparently the following will also work:
lipc-set-prop com.lab126.powerd preventScreenSaver 1
lipc-set-prop com.lab126.powerd preventScreenSaver 0