- Intermittent reward
- Occasional oversized payouts - Quora does this really well. Occasionally, an answer you write will "blow up" and get a disproportionate amount of upvotes. You start to get addicted to discovering the process that caused it to happen but, since the process is essentially random, you continually answer in order to chase that original high.
- Steady stream of accomplishments
- Constant action required
- Appointment mechanics
Probably the best places to look are casino design and advertising design respectively.