To become Jr Member all I need is 30 activities?
Which is 30 posts during a period of 30 days, right?
Yes. After you reach 30 posts, your rank will be set automatically to Junior Member. It's not that hard to make 30 posts. It will take you a few hours if you enjoy posting here on Bitcointalk. I personally, enjoy it.
If someone is not active for a longer period, does he remain the rank or does a downgrade mechanism exist?
Your rank go down if your posts are deleted. It doesn't happen really often unless you post in low quality threads.