There are two common, but not universal, methods I've read about on this site:
1) Losing weight and regaining
2) Heavy/Double Cream based gains.
Personally, I found my belly has been softer after on/off gains with DC starting with a 'harder' belly, although it still feels hard in places with most of the 'soft' feeling lower down, not unexpected with Cream gains judging by other comments.
However I haven't done any significant weight loss/regains to be able to comment on that method.
There are multiple factors involved in gaining such as genetics, hormones, etc. So it's never a guaranty what will/wont work.
9 months