My wine thief would get clogged with stems/seeds, etc when I put it directly into the must. What I do now is partially submerge a fine mesh strainer into the must, then scoop out the juice with a measuring cup. From there, just pour the juice into the thief or test cylinder.
I do just about the same thing that Jim does. Punch the cap down, sanitize a strainer, shove into must, use a sanitized glass or something to get a fairly clean sample. That goes into my test cylinder, which was sanitized also.