Been over a year since alexshel posted this great deal and a nice reminder for those unaware. I've copy/pasted the instructions from alexshel's OP, so thanks to him/her for all the initial efforts. Confirmed working by hm & alz. Stay safe, and enjoy :)

Opt in for 30-day Prime trial. 3 days before trial ends, go to cancel and when a message to change a plan appears, click the button to switch to annual ($59). A day before trial ends, go to cancel and when a message to change a plan appears click the button to switch to annual ($59). Right after changing the plan go back to cancel the plan again - a message to extend the trial for another 30 days will appear. Click the button to extend the plan.

Update 03/05/19. How it may also work: