If your thoughts won't fit on a card, use a typewritten--not handwritten--letter. Letters must be professionally typewritten or word processed--no exceptions. Long, rambling, hard-to-read thank you notes create more frustration than good will.
A thank-you letter is often a sales letter in disguise, so don't be afraid to put some "sell" into it. Send warm thanks to everyone who gives you any kind of help in the job search, however small. No exceptions. No excuses