If a picture paints a thousand words, then a font paints a thousand and one. Or, that's what I believe anyway. Every time I meet with a client, I explain my whole invitation design process and walk through the various steps of choosing the right paper, the right ribbons, the right shade and size of Swarovski crystals. Normally the last question that I spring on the bride and groom is, "Which font(s) do you want to use?" It's always difficult to select from a list of font samples, so I always encourage them to choose a few different ones to compare against each other once everything is typeset for the invitation proof.
For my own invitation, I tried out at least 4 different font combinations before settling on the perfect ones. If you haven't yet given your invitation fonts even a moment's thought, hopefully this quick list will make it easier on you:
1) Uniform or combination?
Do you want to use one uniform font for the entire invitation, or have a combination of fonts? Using one consistent font is great for a clean, simple look. On the other hand, selecting a different font for the couple's names will make the names STAND OUT and DISTINGUISH THEM from the rest of your words. If you're going down the combo route, I would suggest using a clean block print in conjunction with a fancier script style for your names and various headings on insert cards.
2) Formal or informal?
Your invitation gives your guests a hint of what's to come at your actual wedding and reception. Choosing a font that's footloose and fancy free may send the message that your celebration will be fun and casual. Choosing a font that's elegant and tasteful implies a more sophisticated affair.
3) Legible?
You may fall in love with a gorgeous font with lots of fancy flourishes, but it would be a shame if none of your guests could read it. You know letter for letter what your invitation is supposed to say, but your guests won't. Don't make them break out their magnifying glasses or secret decoder rings just to read your invitation.
Have a font-astic time selecting your fonts!