Tracepoints!
Put a breakpoint on a line of code. Bring up the Breakpoints Window and right click on the new breakpoint. Select 'When Hit...'. By ticking the 'Print a message' check box Visual Studio will print out a message to the Debug Output every time the line of code is executed, rather than (or as well as) breaking on it. You can also get it to execute a macro as it passes the line.
