read :
UML 2 Certification Guide ( Tim Weilkiens, Bernd Oestereich) it is a good book and helped vastly improve my UML.
UML2.0 in a nutshell O'Rielley. it has great examples.
Read the UML spec. It adds some details not covered in the above reading.
Presuming that you are working as a coder. try using the knowledge you gained from your reading to improve your UML. Focus on usecase, Package, class, activity, interaction diagrams.
Once you have done all this reading. Go back and do it a second time.
I don't know of any actual practice exams. although brain dump sites are considered cheating on this web site. You may feel tempted to look at them to see if you know UML enough to take the
test.
[ June 12, 2007: Message edited by: peter cooke ]