>>3305371>>3305380There's even special stuff for it (pic), but any non-reactive gas heavier than air would do.
These are your options, ranging fro most to less preferred:
1. make space for air as small as possibe (harmonica bottles or goon bags)
2. drive out any remaining air using Protectan or CO2
3. Refrigerate
Using all three options would give a superior keeping time, but of course it's hard to drive out air from a harmonica bottle whilst pressing it in
