How to protect your marketing ideas?

Over the last year, i have built my business up to a pretty steady level and have been able to work out a few small brand deals. I am now approaching brands I would like to work with, with my own ideas. but have found this to of backfired. I reached out to one brand with 6 new marketing ideas they could use via my audience and they have now gone and used 3 of the ideas for there own good. In a way I feel like I have been ripped off but see it as a lesson.

How can i protect myself when i go to a brand with an idea in hopes of them not just nicking the idea? NDA?

Advice most welcome.

Thanks

You cannot! Welcome to how ad agencies in particular work with freelancers! It is a big reason why I moved onto the internet. You need to provide a solution without really telling them how you do it.

Build a relationship first. Show them that you know what you are talking about. Send them some traffic, show them it converts. Then develop that relationship, there is nothing to stop them from using your idea with others.
Don't try to sell them ideas, try to sell them solutions.