Tell a story. Stories are one of the things that we humans do best, and it's one of the surest ways to keep us invested in something. Every time we see a movie, read a book or watch TV we are seeking out stories. Making a story out of your exhibit will draw your visitor in and make your exhibit more relevant to them.
Keep it short. Avoid writing up huge blocks of text. Too many words will intimidate your visitor. This is not to say that you should sacrifice content for simplicity. Instead, you can use a few tricks to help break up that large wordy paragraph that is so vital to your exhibit. Insert a few line breaks into your block of text and turn it into a series of short blurbs rather than one big, long paragraph.
Know your audience. You need to write to your audience's mental level. If your art center is geared toward a college audience, then your writing can get a bit more technical since your audience has probably studied art. If, however, much of your audience is composed of families with children then you will need to write at two levels, one for the parents and another for the children. Finding out who exactly your audience is will require some study and perhaps even some surveys.
Keep it legible. Avoid fancy fonts that, while pretty, are difficult to read. Bland is not a bad thing in a font for museum text. You may certainly use an eye-popping, inventive font for your title, but the actual text of your exhibit needs to be easily accessible to people of all reading levels.
Work on the format. Make good use of bold and italicized phrases as well as font size. You need to have a hierarchy within your messaging. The biggest, boldest message of them all should be your exhibit's title. Beneath that should be a select number of key points in your exhibit, and beneath those should be some sub-points. Never give a sub-point a more eye-catching appearance than the key point above it.
Make it flow. Each panel of text needs to flow seamlessly into the next. Gaps in information and logic will throw your audience and confuse them. Within any one exhibit you can have several "paths" of messaging, but each should be self-contained enough to inform your visitor without confusing them.
Give it voice. Make good use of interviews, quotes and details. These will work in tandem with the story of your exhibit to draw your visitor in and get them interested.