Ushers are often groomsmen.
Often, you need look no further for wedding ushers than the groomsmen. Typically, groomsmen help with seating guests. However, if the two of you have decided against a large wedding party, or are hosting a very large wedding, you might need some additional people to help escort friends and family to their seats.
They should have welcoming and outgoing personalities.
When selecting ushers, consider their personalities. Because this will often be the first contact guests will have at your wedding, you want your ushers to exude warmth and hospitality. Pick confident guys that are good-humored and talented at small talk. One small detail you may want to mention to your ushers is that they should offer their arm to female guests when seating them. Especially younger gentlemen (and it's perfect fine for your groomsmen to be teenage relatives) might not be as aware of this old-fashioned gesture.
For a big wedding, consider selecting a head usher.
If your wedding venue is large and you are expecting quite the crowd, you may want to put one usher in charge. Other ushers can confer with him if there are issues (Aunt Betty thinks she ought to be sitting in the second row, etc.), and he should be the one to manage your notes or seating chart. It's also wise to select a relative for this role, as he can more easily recognize some of the "key players" at the event.
Make sure to give the ushers clear instructions or a seating diagram.
Unless your seating is very relaxed, you will want to provide your wedding ushers with a list of preferences for seating. Designate how many rows are blocked off for relatives and what types of relatives should be seated in each one. For example, although you don't need to assign specific seats (you certainly can, if you prefer), you might have your parents and siblings in the first row, with aunts and uncles in the second row, etc.
Give them a gift.
Even if your ushers aren't groomsmen, it's still nice to give them a gift as a token of your appreciation. Although, it should be noted that they do not need to be in the wedding portraits with the rest of the wedding party, unless you would like for them to be included.