The 3 keys to unlock the door to usability

There are three keys to unlock the door to usability: frequency, sequence and importance.

Frequency says that things the customers do most frequently (e.g. next, back, search etc) should have a prominent position in the sequence.

Sequence says that activities that occur in sequence should be presented in sequence (i.e. you pay at the end of a transaction, not in the middle).

Importance means that important pieces of information need to be given clearly and at the right time (e.g. if you only ship within the EU, then a customer trying to buy from India needs to know this early on –not at the end of a six-page check-out dialogue).

Understanding the customers’ mental model and applying the frequency, sequence and importance rule will crack most of your usability needs. But, beware, like all rules, you cannot follow them blindly, and there are always tradeoffs that have to be made between these elements.
This is Service Design Thinking