I started using fonts when I did calligraphy and this became a foundation for how I use different fonts now, particularly on my greetings cards, although I very rarely use a calligraphy pen and ink these days.
Whenever I want to add a sentiment to a card, my favourite place to go is Word. As anyone with a computer will know, there is a whole list of fonts ranging from the simple to the cryptic. Essentially, there is a font for every occasion which is why I love it.
Calligraphy required quite a bit of patience and time: once you've chosen the font and size, the lines need drawing up, I tend to pencil my text out before inking it, then the ink has to dry, then the pencil has to be rubbed out. This system, however time consuming allowed me to create text that was neat and uniform and I used the same system today, even if I don't use the calligraphy pen.
When I use Word for writing sentiments, I find it easiest to actually write out what I want into the document and copy it onto the card, occasionally drawing up some guide lines in pencil first.
I try to match the font to the theme, using more calligraphic ones for serious cards and looser, fun ones for less serious cards.