I have seen several different methods--I am not in favor of any of them.
Silk, and the dyes used in ribbons both decompose over time. All you can really do is retard the deterioration. The absolute best way to preserve antique medals is to lay them flat and keep them in the dark. If possible, limiting atmospheric exchange (placing them in archival envelopes or a Riker-type mount will also help slow the decay.
If you insist on displaying 100 year old medals hanging on uniforms, I have seen two methods that when carefully and correctly executed, showed moderate success. I note though that this is at your own risk and I do not guarantee positive results.
1) Purchase "jewelry U-pins." These are a sort of U-shaped pin you can carefully use to support the planchet so that it is not putting stress on the ribbon drape.
2) Use fishing line passed through the metal ring and the brooch--being careful to tie it off in such a way as to have the weight of the medal resting on the fishing line and not the ribbon.
Even though I have seen the above done successfully, I still don't recommend either one.