This may seem intuitive, but I'll post it anyway.
With the PLEX now as a reward for the buddy program, you can essentially create ten 51 day trials a month.
1) Send a buddy program invite to yourself or someone you know,
2) Give them PLEX or the money for the PLEX
3) Use the PLEX on the trial account and upgrade to a full account + 30 days
4) Receive a PLEX as a reward on your main account.
5) Sell the PLEX back on the market for virtually no net loss