Here's my advice for "secondary" DNS service. I recommend running the master unlisted ("stealth master") and using it only to serve zone transfer to the slaves. It can also be a good idea to have a backup "stealth" slave that could become the master.
I call them "slaves" even though, in registration terms, I think they're still called "primary" and "secondary." I have yet to find a practical distinction, and, with a stealth master, there could be confusion.
Make sure to have at least one slave listed from a different TLD (.com, .org, .net, or a ccTLD).
A list of my preferred providers, reasonably priced:
DNS Made Easy (per 5-10 million query pricing)
BackupDNS (flat per zone per month)
EasyDNS (per million query pricing)
DNS Unlimited (cheap per million query pricing)
Durable DNS (per million query pricing)
No-IP "squared" (flat per domain per year)
Not all of them support configuring more than one master, but they all have web access to effect the changes.
More detailed advice may be forthcoming.