With multiple upstreams, you can influence traffic paths.
Outbound traffic (you control): Use LOCAL_PREF to prefer one provider for destinations.
Inbound traffic (you influence): You can't force others to send traffic a certain way, but you can hint:
AS_PATH prepending. Make one path longer.
Selective advertisement. Advertise some prefixes only to certain providers.
MED. Suggest entry point if both providers are same AS.
Communities. Some providers offer communities to adjust LOCAL_PREF.