The really simple answers:
0) Start with the page templates at [mobi...] (developers area). In particular make sure that you keep the correct XHTML MOBILE declaration line at the top of every page you generate.
1) Test your site on the simulators at [mobi...]
2) Test your pages using the ctl-alt-V built in validator in Opera: it will use the declaration line from (0) to get your page validated correctly at w3.org, even pages that are not up yet.
3) View your pages using the "small screen" mode of Opera. Also test in FF, which handles XHTML. IE6 doesn't (usually) in my experience.
4) Use a real mobile phone.
5) Get one the of the UP/OpenWave simulators (gee, I knew them when they were still UnwiredPlanet and we were trying to launch the FIRST mobile price-comparison service in the UK). I think this is the URL you may want: [developer.openwave.com...]
[edited by: DamonHD at 7:19 am (utc) on Oct. 18, 2006]