How can I prevent people from seeing when I'm online?

Try this: Settings > Account > Privacy.

Then you will see the Last Seen option.

Thanks. I set it to "Nobody". Then tested it with my other phone and it still shows that I'm online.

What am I doing wrong?

Normally, if you set your "Last Seen" to Nobody, it will no longer appear for your contacts. This is a workaround, and there is not really an offline mode on WhatsApp. Alternatively, try opening WhatsApp with airplane mode on.

Yeah, just confirmed it several times. That Nobody setting doesn't work. It still shows "online" as soon as I open WhatsApp, and if I don't even go into that person's chat. Also setting the phone into an airplane mode doesn't help because then I can't use WhatApp. What's the point?

Thanks for trying to help though. It seems like anything else with facebook it's just FUBR.

The airplane mode will only be useful to read your messages offline. To load new messages or to send one, it will no longer be helpful. There are no other methods at this time to appear offline...