I had a field day yesterday taking advantage of New York WebSphere User Group free certification testing opportunity - passed 252, 142, and 665 tests - three in a row. I did not get very great percentage, but I passed and that's what counts!
With 252, concentrate on security and admin/deployment/ASTK. With 142, make sure you know DTD and XML Schema as well as XSLT well. 665 - fairly easy to pass, as long as you are familiar with IBM's product offering and SOA guiding principals - you'd pass. Reading WebSphere SOA redbook is also helpful.