Hi oldleclericrd,
I've never had an opportunity to do it but if the antlers are still in the growing stage you have to inject them with formaldehyde immediately until you flush all the blood out of them. Hang them upside down and and with a small hole in the tips flush clean. From what I have heard a lot of work with mixed results. There are some safer chemicals out there to inject with but never have used them in this way.
If they are hard horned and pretty much stopped growing there are a couple options. I took two deer in from Montana last year that were both still in the velvet in November, stags. You can have them freeze dried which I've heard is kind of expensive. On these two I left in my freezer for about a year then saturated them with a velvet tan liquid I purchased from one of my supply catalogs.
One other option is to strip the velvet and have artificial velvet put back on. Kind of expensive too, depending on the size, but you don't have to worry about decomposing ever. I have a set of mule deer antlers in the shop now that we had done, they look pretty darn good. Any questions give me a call. Rick