Well doing your job as a project manager will give you 15 pdus (5 PDUs every year).
You can also do the following:
- Write articles and get them published on a decent and recognized project management website
- Speak about project management in a seminar or something
- Write a book about project management (not an easy thing)
- Do some volunteer project management work
- Reading and enriching your project management experience
Check the PMP handbook for more information on how to get PDUs for free