Content is King
Sure, we've all heard it, but it is getting more and more critical the smaller mobile devices get. Fact is that there really isn't space for any unnecessary design elements. This forces the designer to present the content in a more appealing way. This could be by using common-sense layout techniques or by using the device's inherent display features. The obvious example is the Apple's (and now most other OS's) pinch & zoom type features.
It's not what they see, it's how they see it
User experience is key when designing for mobile devices. The user needs to be guided through the app/site without them becoming frustrated with the process. The aim is to define a path through the site and lead your visitor to where you want them. Having said that, make sure that they can easily navigate to pages they know they want to see.
Advertising can be dangerous
Everybody wants to make a buck, but be careful not to let your ads interfere with the user experience. As mentioned before, screen-space is at a premium, use it wisely.
Make it usefull
This actually goes for most things. The last thing a user wants is to have his time wasted. Decide what you want to achieve and work towards it.
