Set break points in my Java code
Start up the Java application server with the appropriate debug JVM properties set (e.g. -Xdebug -Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=n)
From Eclipse, I attach a remote debugger to the app server on the default port 8000. The Java Debugger will open up when a break point is hit.
Set breakpoints in my Flex application (or one of its modules).
From Eclipse (with Flash Builder) I launch a debug configuration for my Flex app. The Flex Debugger will open up when a break point is hit.
At this point I have two debuggers open and everything work great. Two other things I do:
a) extend the transaction system timeout, so it doesn't get trigger while I am sitting there think for a few minutes
b) use Charles Proxy (in reverse proxy mode) inbetween the client and server to watch the AMF traffic and view payloads, etc.