Service based model
This is not a very new concept (http://en.wikipedia.org/wiki/Service-oriented_architecture) but the cloud model makes this very important. Build your business model such that it can be consumed as a service. This would also force you to modularize parts and all this would ensure you have a high degree of portability.
Build for failure
Cloud is multi-tenant in most cases and with it comes challenges like noisy neighbours or failure of individual components. Build for these scenarios. Symian army (http://techblog.netflix.com/2011/07/netflix-simian-army.html) talks a lot about this and is an interesting read. Importantly plan for "What happens when"
In building for failure you are also creating a good recovery model. One the benefits of running everything as code means that you can recover faster and this would translate into better uptime.
Cloud is all about the API and pluggabiliity. Think about building a top level API for your business model. Then use vendor APIs and plug them into your API. Wherever possible, loosely couple your application interaction. For e.g., instead of direct database calls use an API
Monitoring becomes more than just making sure your applications are working fine. If you leverage multiple cloud providers, you can use it to make operational decisions. You can use it to go with the best cloud provider and save costs. You can use it to find low performing instances within the same provider. One important point here is to make sure your monitoring is vendor agnostic and wherever possible not a tool provided by the vendor. Frameworks like Sensu (http://www.sonian.com/cloud-monitoring-sensu/) or tools like Riemann(http://riemann.io) can help
Cloud will force automation to a large extent and you need to embrace it. Automation also allows you to build across different vendors When using multiple vendors, us a model that works on all platforms. There are open source libraries like libcloud which provide vendor agnostic ways. Be careful with using automation providers as you can get vendor lock-in in a different way. While building your own autoscale model is complex in the long term, there is a lot more to gain as it will fit your business model.
Think about data
Cloud provides commodity based services for things like compute, storage etc but your data is not commodity. So think about distributing this over different vendors or build that into your recovery model.
Think about security
Security in the cloud is a hot topic and it is safe to say that this is still evolving. This is also something that is overlooked while you plug in other nuts and bolts. Make sure things like identity management, access control models are at the heart of your cloud strategy. Even if security is not an immediate requirement, you can build them as services which can be implemented at a later stage.