It basically boils down to lowering the RSC's size and improving your internet connection/upload speed.
Your options include:
-Use less resources.
-Clean compiling removes unused resources from the RSC file.
-Use smaller formats for them. eg PNG instead of BMP, OGG instead of WAV...
-Host some resources on an external server.
-Host the whole RSC file on a decent external/separate server, using preload_rsc to make the players download it from there.
-Zipping the RSC manually with a program that supports slightly better compression rates might work (BYOND might not be able to unzip it).