Because I was born a girl and have played the harp for more than 25 years as a woman, this is primarily a guide for lady harpists. I will say a few words first to the men and get them out of the way, as it were.
 
Guys, the proper attire could be anything from a tuxedo or dinner jacket to worn jeans. Ask the people in charge of the event if you have any doubt. If you have no idea and no way to find out, you can't go far wrong with a dark suit and tie. If the jacket hampers your performance, take it off! Your music comes first.
 
If called on to wear a kilt "like the ancient bards," that's fine - if you play a lap harp like the ancient bards. If you play a floor harp, you might remind your hosts that the ancient Irishman didn't wear a kilt but a long, shirtlike garment called a leine - and a man with the dignity of a bard wore a long leine. Then opt for a pair of trews.
 
Seriously, it is possible to play a floor harp gracefully in a kilt, but it must be done with care. Read the rest of this guide, and practice sitting with your harp before a mirror.
 
On to what I know best: performance attire for the lady harpist. When creating your performance wardrobe, always keep in mind that you will be holding the harp with your knees and want to do so gracefully.  Wear a skirt, even for casual events: pants will pull at the hips and thighs, which is far from flattering. The skirt should be long: mid-calf at the very least - and above all, it must be full enough to accommodate the harp without pulling taut over any part of your anatomy.
 
The style should be dramatic but subtly so - open laces at the neck of a close-fitting midnight blue boucle sweater and matching skirt, for example. Unless you are performing for the SCA, it should not look like a period costume. Your clothing should enhance your performance, not overpower it.
 
Some harpists wear black for all performances, even weddings. If you do so, you've already read all you need to know. Personally, I would never wear black at a wedding, and have a wardrobe of performance garments in most colors and from formal to casual.
 
You should have at least two formals in colors that won't clash with your wedding clients' color schemes. My two favorites are in Wedgewood blue and cerise (a red similar to burgundy but lighter), and one of these will harmonize with most weddings. In the unlikely event that neither of them works, I can always wear something ankle-length and dressy.
 
It may be hard to find a new formal with the requisite full skirt. The full crystal pleats popular a few years ago are perfect, so go vintage. You can find the right vintage gown, like new or NWT, on eBay! But remember, sizes were different a few years ago. I wear size 4 or 6 in present-day dresses, but my vintage formals are size 9/10 and fit as though they were made for me. You may want to buy one at least two sizes larger than you normally wear, so measure a well-fitting dress to see what you need and ask the seller to measure the dress for you.
 
For less formal occasions, you should have a few good skirts that are full at the knees and ankles but not necessarily at the waist. Ankle length is in fashion now and is perfect for the harpist. Full broomstick pleats are wonderful, and so are tiered skirts and full A-line knits. I have tiered skirts in everything from velvet to washed cotton - from very dressy to casual - and have never been caught with nothing to wear.
 
Velvet is wonderful for performing clothes, especially around the Christmas season. Here is a picture of me in a blue velvet top and velvet and plaid tiered skirt that has been a favorite performing outfit for years. I sometimes vary it by adding a blue velvet silk burnout shawl I bought on eBay in 2003 from viccap99.

In this picture, my left foot is right beside the harp, although a reflection in the harp's side makes it look farther away at first glance. My right foot is on the other side of the harp. The skirt is full enough not to bind in this position but to fall naturally, and long enough to cover my legs. This is a good effect to strive for.
 
A little glitter in your outfit will also give you a special feel. One of my favorites for Christmas is a long, full (of course) black knit A-line skirt, worn with a black velvet top with metallic gold tracings and a subtle red and green floral design. Wear whatever is flattering to you, keep a variety of colors, and always remember: "long and full."
 
Wear comfortable shoes with flat or only slightly raised heels: higher heels may distract you, and if you get a low chair they may make your knees stick up like a grasshopper's. I have a pair of comfortable black leather pumps with a low wedge heel that I wear for all performances and only for performances - they are visible in the picture above. I am all but superstitious about them! Whether you develop a favorite pair like this or not, resist the temptation to perform in heels.
 
Undergarments? Comfortable and, in the case of slips, full! You may be able to find a half slip with at least a half-circle fullness; if not, make one or have it made for you.
 
Wear jewelry, but avoid anything that might interfere with your performance. No bangle bracelets (chain bracelets or close-fitting stretch bracelets are fine, and their sparkle may enhance the experience for your audience). People will probably watch your hands. Wear rings, but only if they will not bother you. (I wear rings when playing background music only.) Nail polish is great, and you might want to get some that sparkles or even wear a little glitter on your hands and arms.
 
Most of my observations have come from trial and, definitely, error. I've discovered the errors in photos taken at performances: skirts that pulled around the hips; grasshopper knees in high heels that looked almost indecent. Benefit from my deplorable mistakes! 
 
Blessings!
 
Shirley Starke
harpistnd