One of the most best ways to gain in real life is share with someone who will share what they have with you. It's a way to build up worth without losing anything.
Advertising your channel for free is extremely easy.
On every channel you are offered the feature to put other people's inside a box which you can place anywhere you want.
So here is what you do: Find someone that has equal amount of subscribers than you, maybe more or less, get them to put you in their box, and then put them in your box. You can agree on the amount of time.
Tips:
Look for people that have the same niche as you.
Don't make any long deals. Eventually you will stop getting traffic from a box. Find someone else.
Don't try and scam someone (Not putting them into your box). If you read my last post I talked about keeping a good reputation in the YouTube community.
