Asking for Referrals...

 In my experience an email works best.  This gives the client the opportunity to reply at their convenience and saves them being put on the spot by a telephone call.

I tend to mention the possibility of referrals when I am either with the client or over the phone (usually when I know the client is happy with the service provided).

I wouldn't just fire off random emails without first discussing it with them first.

Suppose it depends on what your relationships are with your clients.

Referrals are worth the effort, much less selling involved and less focussed on getting the fee down.

Gary