I give away a 2 week free trial with no strings attached. No credit card info necessary - just give me your email address. Half way through the free trial, subs

I give away a 2 week free trial with no strings attached. No credit card info necessary - just give me your email address. Half way through the free trial, subscribers are asked if they'd like to become a paid subscriber. Once the free trial is over, I then send a bunch of automated "hey please subscribe, pleaseeeee" emails at 14, 15, 17, 20, 23, 28, 31, 35 and 40 days after the subscriber's trial ends. Currently there's a ~20% conversion rate from trial to paid. Used to be around 30% for a while, but as more strangers - i.e: people that don't follow me on Twitter, don't know me from other sources such as my freelance tech writing or podcasting - sign up that's dropping slowly. Getting people to sign up for a free trial is my main focus and success there comes in ups and downs. So many hours wasted on things that don't work, but every now and then something unexpected goes well and I get a spike in people signing up for the trial (and hence, become paid subscribers in 2-3-4 weeks after their trial ends).