The following tips are mainly for folks who are having a more informal portrait done, such as an engagement portrait or a family portrait.

Be comfortable

We photographers often start by telling clients to wear something they are comfortable in, both physically comfortable (i.e. don’t wear pants that are too tight) and personally, mentally comfortable (i.e. if you haven’t worn a suit since your grandfather died ten years ago, don’t wear a suit for your portrait). Wear something that is you, but do be aware that the camera can be unforgiving, and while casual is okay, careless probably is not . “Dressy casual” sums it up nicely most of the time. 
For engagement portraits, consider wearing what you might wear on a casual date. We live in a very casual age, and blue jeans, khakis or chinos are fine. Wear comfortable shoes. Women, be aware that I don’t usually get your feet into personal portraits, and for engagement portraits, we may walk around a bit. If you plan to wear heels, be sure to bring some flats to walk in as well.

Avoid flashy clothes, patterns, and very strong primary colors

This is standard advice you’ll find everywhere. Avoid wearing clothing—shirts, hats, sweaters, pants—that draws attention to itself and distracts from your face. Stripes (like rugby jerseys) and shirts with striking patterns (Hawaiian shirts) are generally no-nos. And strong colors (red, yellow, green) can also be difficult to deal with photographically, unless you know we’re going to do a black and white portrait. It’s an old photographer’s guideline that darker or subdued colors are slimming and also easier to deal with than bright colors or whites, but don’t feel bound by it. If you have a white linen dress that you love, by all means, wear it.

Cover up

Avoid “short”: short sleeves, and especially short pants and short skirts. Bare skin can be distracting.

Makeup and grooming

Hair and makeup are difficult subjects. Give your hair some thought and at least bring a hair brush with you. Men, ask your women about your hair. A haircut a week before your portrait is often a good idea. As for makeup, if dramatic makeup is your style, that’s fine with me. But I suggest to most women that they try to tone down their makeup for their portrait. Less is more. Don’t worry terribly about tiny blemishes: I can remove them after the shoot. (You may want to apply a little makeup over a blemish simply so you feel more comfortable and less self-conscious during the shoot.)

Groups: Dress alike, but not too alike

Couples posing for an engagement portrait and families posing together should make an effort to dress so that everybody harmonizes with everybody else—but I generally don’t suggest wearing a uniform. If the woman is going to wear a nice dress, the man clean up, too. But this is a difficult issue and hardly a rule. If you have identical twins, for example, and want to dress them identically, that’s perfectly fine. However, a family portrait in which everybody wears blue jeans and a red sweater may not work as well as you want: viewers may end up seeing nothing but the jeans and sweaters. Try to look natural.

There are few rules

In spite of the very traditional tips above, I want you to know that I know that there are really no hard and fast rules. If the woman feels comfortable in a sleek DKNY number with gorgeous pumps and loop earrings, and the man feels comfortable in jeans, tee shirt and tennis shoes, that’s cool with me. These days, heaven knows, anything goes. However, it would be a good idea for the man to bring a blazer and perhaps a pair of leather shoes in a paper bag, just in case.

Choices are good

Final note: It’s good to have choices. If you dress in layers, we can easily take something off if it looks better. If you would really like to have choices, bring a change of clothing.
And remember to bring your sense of humor and have a good time!
By the way: the tips above are pretty standard. You will find more advice of the same type by searching on the Web for “what to wear for a portrait” or “how to dress for an engagement portrait”.
This article is a rewrite of one I posted at my old blog two years ago.