Default switches may be added to the Windows environment variable, RAR, using the SET command. Most of switches specified in this variable are used only when WinRAR executed in the command line mode. Some switches can be applicable both in shell and in command line modes.
Command:
SET RAR=-s -m5 -md64m
will force WinRAR to create solid archives using best compression and a 64 MB dictionary when executed in the command line mode.