Cross post from dr TLM on aunt minnie.. i think it directly answers some of our questions:
The most reliable way to convey to a program that they are your #1 choice (and have it make a difference in their ranking scheme) is to have one of your radiology letter writers (the bigger name/clout, the better) call your #1 program's PD and tell them that you're ranking them number one.
There is more credibility because a well-known attending is willing to put his/her name "on the line" by vouching for you. When an applicant says that they are ranking a program #1, it means very little because PDs will suspect you are telling multiple programs the same thing (most PDs - and applicants for that matter - have been burned and are therefore suspicious). When the PD at UCSF or vice chair at MIR calls the PD at UCLA to say that you will be ranking UCLA No. 1, it's understood that UCLA is the only place getting that phone call.
Naturally, this puts a kibosh on your politicking, e.g. you can't play both sides of the field and your home institution will know that you are not ranking them #1, but that's the cost of being credible. Whether the added benefit from such a move is worth the potential loss of opportunity (e.g. at your home institution) is another issue: If you're a superstar applicant and UCLA was gonna rank you #1 to begin with, you've gained nothing. If you weren't seriously considered by UCLA and your home institution was your best backup, you've gained nothing and possibly lost your best backup.