Being a multihomed ISP with BGP-speaking customers isn't very different from just being a multihomed network in most regards, but some things need extra attention. A regular multihomed network sends out just its own routes, which makes for easy outbound filtering: allow out the BGP announcements you're trying to send, and nothing else. When you're providing transit services, it gets a bit more complex. You can, of course, filter out any announcements that don't fit your idea of how your customers should announce their routes. That way, however, you don't leave any room for traffic engineering or BGP-based anti-DoS measures. A better solution is to accept all reasonable announcements. You should also have a solution for customers with two connections to your network. And expect customers to come to you with questions about IP multicast and IPv6.