Start with having a different signup for desktop and mobile version. Then you can start testing on what's working better. Flyouts or popups? Once you have those numbers, then you can start testing which offers works better.
Test everything to improve conversion. Test: - images - 10% or 15% discount ? - call to actions - colors - frames vs no frames - using personalization - using different copy
Sometimes it takes only one simple change to improve the conversion.