The best practice in every project is cyclic prototyping; start prototyping very early. Very quickly, all design aspects and provisions to be made will reveal themselves. Repeat this process until necessary.

In prototyping consider the following rules:

1. Recognize That Ideas Are Cheap – Given the connected, Internet-savvy world in which we live, ideas have become cheap … A great prototype is often the best way to start a dialogue with potential customers and test your idea’s value.
2. Start With a Paper Design – … Writing code without real consideration for several design factors leads to heartache and a lot of rework … You can get peers and, hopefully, customers to give feedback … Paper designs are inexpensive and more valuable than words.
3. Put in Just Enough Work – Know your objectives and stick to them … Keep these objectives in mind and be careful not to fall in love with the process … you want to invest just enough time and work to meet the objectives.
4. Anticipate for Multiple Options – Design your prototype with modularity in mind … Customers ultimately decide how to use your product, not you. Design in options for expansion, performance, packaging, and lower cost.
5. Design for Reuse in the Final Product – … Look for prototyping tools that make it possible for you to scale your prototype from lab to market.
6. Avoid Focusing on Cost Too Early – … Initially, focus on proving the value of your innovation, and design with modularity in mind … Focus on securing your first set of customers and then work on cost optimization.
7. Fight “Reversion to the Mean” – When prototyping, the tendency is to develop something easy rather than develop something that has a “wow” factor. Stay true to your vision and make sure your prototype captures the original thought of your innovation.
8. Ensure You Can Demonstrate Your Prototype – … Do not build up to a crescendo. Most people’s attention spans are limited to less than 60 seconds … If the demonstration is amazing, all else falls into place.

[source] http://zone.ni.com/devzone/cda/pub/p/id/579