Careful with the diacritics
I am always happy to see modifications done to fonts in use, but only if they're good. Here in Slovakia we use a lot of diacritic marks and not every font has them, so it's very common to see really terrible examples of 'handmade' marks, even when created by serious design agencies. Bad diacritics spoil everything.
Draw your own inspiration
Using display faces can be more like illustration than typesetting. Research and find examples of a style that inspires you. I find it easier to draw the shapes and styles that I want my header text to look like. If the font you want doesn't exist, create it yourself.
Think about the company your type keeps
With all display faces (like any other good typeface), the designer still plays an important role in setting them up. Poor kerning or layout can easily turn a beauty into a beast. Another factor when considering a display face is it's pairing [with other fonts]: a factor that can really make or break your selection, and occasionally even showcase its qualities even more.
Clarity is vital
This is important. Very important. How well does the typeface read at the intended size(s) and distance(s)? Now that you've answered that, ignore it and ask someone else to look at it. Can they actually read it? Just because a typeface 'looks' cool doesn't mean it will 'read' cool. Did I say it was important already?
Text to display
There is one area frequently overlooked: the display potential of workhorse text typefaces. There are Thin and Ultra Bold (or Black) versions of text typefaces crying out to be used large. These styles are at the extremes of the type family and not meant for normal running text. They often capture the essence, the uniqueness, of a typeface.
Mixing it up
For me one of the most interesting things to try with a headline font is to use it out of context. Try selecting a font that is familiar to a specific use or type of design, and use it for something completely different. For instance, select a font that is more familiar in pub windows and try it for an exhibition.
A good display font gives a more playful, daring, sexy and emotional feel to your design. If you are frightened by these words, then don't use one. Use a display font in the same way that you would add spice to your food. Very much like a person, every typeface has its own character; display fonts are just a little bit more extrovert, and that's OK.
Careful with connections
Many OpenType fonts - especially scripts - depend on Contextual Alternates and/or Ligatures to automatically set correctly. Check the OpenType menu or palette in your app to make sure these options are active. Never ever use the 'optical' spacing setting in Adobe applications with connecting scripts - it will completely mess them up.
A note on fashion
Like footballers, it seems over the last decade type designers have started to acquire fans of their own. They say the player is never bigger than the club, but in this case it seems the designer is usually bigger than the foundry. If you've accumulated a good following of supporters and fans towards your work, this can have a huge effect on how your designs perform regardless of technical ability or experience.
Context and creativity
As with all design, there are a lot of things to think about when using display fonts - not least being appropriate to content and context. The challenge is to be exciting, dramatic and eye-catching, and the best way to do this is to try to be original. A new and different message deserves to be presented in a new and different way.
Check out this fantastic selection of free fonts