From my perspective if you need to do two things and signups will follow. 

Do these and you should be fine. 

And I don’t think there is much more you can do to effectively push your numbers up when it comes to developer signups. 

Show people real value your product will give them

You make those examples realistic, let people experience that value that you claim to have. 

Populate sandboxes with data that looks as close to their use case as possible. Figure out how would they populate it if they had to do a test/POC of your product. Show that. 

You do that, but actually do that, and I believe you are 80% there. Devs will jump through hoops when they believe you can deliver.

That is the hard part IMHO. Building a realistic experience. Making devs believe that this could be their use case. Focus most of your time here.

You can put a demo on top of it to guide people through this experience. This could be good or bad depending on your audience.

But to push conversions up you should probably do one more thing.

Make it clear how to reach out to you

You want to make it as obvious as possible how to convert. Adding clear buttons contact, trial, get demo, help, etc will get folks to reach out when they do get stuck.

Those shouldn’t pop up as they will get closed immediately. You should rather make them visible and accessible for when it is needed. 

And most of the time there will be a moment when devs will want to reach out, if not to you then to the docs or resources. 

And... that's it afaik. Not much else you can do. 

  • Your product sandbox should make people interested, motivated. 
  • Your CTAs should take some friction away letting them know what to do. 

If you don't see conversions from that my bet would be on the product tour/sandbox not showing value to the group of people you are going after. 

Good examples

I liked how Vercel implemented their product tour conversion-wise. Clear CTAs to take the next step. 

Developer Markepear testimonials
Read full break down of this example

For the sandbox specifically, Sentry does a good job IMHO. They do go for a gated modal which can go both ways but overall I like it.

Developer Markepear testimonials
Read full break down of this example