Either way is o.k. If it is typed, don't sound too formal. I got one last year that read like a court summons. (wait a minute...
)
Seriously, the people getting them understand you are just playing the game for the most part. They all played it too. You still get points for playing it however.
If you honestly had a good time and honestly see yourself ranking them highly, tell them so. Interviewers are human too. They tend to want people who want them.
I've recieved a few at least a month afterwards. Many will tell you that is too late. For a gift I guess it might be. But a little later does help refresh the memory of the inteviewer. On that note, it doesn't hurt to send a note promptly and then call the program a month or even more into the interview season to touch base and gently remind them of your existence and/or your desire to match there. There is nothing against the rules or unethical about telling a program you intend to rank them highly or even number one. It is just wrong for either party (programs really) to ask this information.
Try to have all of your follow up contacts completed by February 1st. Many programs begin finalizing rank lists by then.