Personally we do the following:
custom:
defaultStage: dev
profiles:
dev: dev
prod: prod
test: dev
provider:
stage: ${opt:stage, self:custom.defaultStage}
region: us-east-1
profile: ${self:custom.profiles.${self:provider.stage}}
Setup multiple AWS accounts as others above have mentioned and then you just need to specify the stage and it automatically will select the correct profile to use; dev and test use our dev acc, prod the prod account.
Tailor as you see fit,
Ryan