Update - Kristoff was asking about using Eloqua for retention - not how Eloqua retains customers: For traditional use cases of Eloqua, you will obtain insight into which customers are engaged/fans of the marketing communications and web messaging. If you happen to have a digital product or be using digital for things related to customer success, training/enablement, community, support or automated billing notifications, you will get insight into how your customers are engaging with things that trigger risk.
I can't speak for Eloqua, but I have a couple of thoughts of my own (sorry tried to keep this to three - failed):
1. Understand in app engagement (products like Totango help flag at risk clients by login counts and features used in SaaS).
2. Be clear about renewal and billing - love this thread by Quora - What can a startup do to get their customers to pay on time? - Quora
3. Tier accounts and prioritize support/customer success/consulting.
4. Be respected as a thought leader by producing awesome content/research to help your customers.
5. Build an awesome product that people want.
6. Get customers talking to other customers (Events/Topliners).