In today's modern society, it is not always appropriate for a minister to wear full ceremonial robes to officiate a wedding. Depending on what the bride and groom request, the minister can wear full robes, or something much more casual. It depends on the feel of the wedding, the spirituality of the couple, and the couple's wishes.
Religious Ceremony
If the wedding ceremony is considered to be a religious one, then a minister should most likely wear his or her traditional robes. Speaking to the couple beforehand can tell you whether they are expecting a more traditional, religious ceremony. If they are, it's best to choose the entire traditional garb that you would wear for a church service, depending on the religion. However, if you speak to the couple and they indicate that they'd rather have you wear a suit or something else, then you can accommodate them within reason.
Non-Religious Ceremony
If you are performing a non-religious or non-denominational ceremony, then it is up to the couple, and your comfort zone as to what you should wear. Most ministers will choose to wear a suit and tie if it's a non-religious but still formal wedding. A female minister should wear a modestly designed and colored dress. You should refrain from wearing a white dress or white suit, so as not to distract from the bride. Speak to the couple and ask what their expectations are about what you should wear.
Casual or Themed Ceremony
If the wedding ceremony that you're performing is more casual, like a beach wedding or a wedding taking place in Vegas, then you can probably wear something a little more casual. It might be appropriate to wear a linen shirt and pants for a beach wedding. If the wedding has a theme, such as a Western theme or a Halloween one, talk to the couple; they probably have an idea of how they'd like you to dress. Make sure it is within reason, and doesn't take away from your authority while officiating the wedding.