The trick with having a wedding party is that eventually you're going to have to choose someone to be the best' something. Women have it slightly easier because they can opt to have a Matron of Honor as well as a Maid of Honor, but even so, it's a difficult decision to make.
Many women feel the pressure of having to choose their bestest' friend for the honor of being Matron or Maid in their wedding. They feel like they are leaving one of their friends out or hurting their feelings. First of all, realize that the duty of being in the wedding party is enormous. The Matron of Honor has a lot to do before and during the wedding itself, so you will want to choose someone that you think can handle it.
If you're the bride, you may feel that you have to choose a family member as well, but that's not necessary. You might include them as a part of the party, but if you have a really close friend that you want to bestow the honor upon, then do so.
Find someone who isn't overly busy or committed. If they live in the town that you'll be married in, that's great too. Even if you're out of town, you'll get to be able to visit each other and she can make arrangements while you're at your home. You also want to find someone who isn't financially burdened. The thing is that gowns, showers, bachelorette parties, and gifts all cost money, so you don't want to stretch them.
And then the men
Just like choosing the Matron or Maid of Honor, you don't want to choose someone that should' be the Best Man. This should be someone that you can count on to plan the bachelor party and get the other groomsmen to stay on schedule. Of course, your best friend may fit this bill, so the decision might be easy. Again, don't pick someone who doesn't have a lot of time or is financially strapped.
Picking someone to be your right hand person is an important decision and an honor for the person that you ask. Although you may think that they are the right person, they may decline the offer and the extra work. In that case, you will need to find someone else to fill the role.