<p>I am creating a mobile web application using asp.net. The application must support iPhone, Blackberry and Windows Mobile Platform. What are the low things which should be taken care of while developing this application. Is there any best practices list available for this?</p>

<p><strong>Summary:</strong>  </p>

Can you add some more info? If you are developing a application for all those platforms, I would guess you are creating either some sort of web-services or just a "standard" web-application?
If it's just a standard web-application, I think it's a bit hard to ensure a good compatibility due to the fact that javascript, which is widely used in webapplications isn't always implemented in browsers for mobile phones.... Yes, if you have Opera, and not Pocket Internet Explorer, it get's a bit better. When it comes to Blackberry I'm not quite sure how well the browser performs.
Iphone on the other hand is probably "easy" to develop for, since it's default browser is probably good at javascript - just as Opera mini.
Also - Other graphical issues due to the fact that many Windows Mobile devices have a reather low resolution should also be taken in consideration, but this is eventually getting better with the new devices beeing released :) (As Touch HD, Omnia, Diamond etc)