Contact e-commerce websites like Amazon, and request that you want to pay with Bitcoin, Ether or any other coin you like.
It can be done by using site's feedback or contact us forms.
This way we can create demand because good e-commerce sites care about their users. And they know that the more payment methods you have the higher the conversion rate (the % of people who buy something) because some users can use only 1 method of payment. For example, some americans can only pay with Paypal while some venezuelans can only pay with Bitcoin, etc. So, if the site supports more payment methods it will convert more orders on average.
To increase the demand for implementing cryptocurrency payments in e-commerce sites we just have to request that using the contact us or feedback forms. The more people give feedback to their favorite online retailers the more people will start using bitcoin as payment method (now that we have segwit).
The more people do this the more demand for Bitcoin there will be.
Thanks to those who do that.
I'm not quite sure if this idea can increase bitcoin demand. Because in the first place, people who have not a single idea about btc will just chose cash/credit cards for payment on their purchases. The best way to increase demand is spread the word and educate people about cryptocurrency or bitcoin to be specific. How it works and what are its uses and advantages.