These are extremely important. Be sure to go for glowing over glitz. In other words, avoid obtaining a recommendation from that CEO friend of your father or other person with a fancy title if that person does not know you well. Instead, find someone who you know will provide you with a glowing letter of recommendation that shows that you are a person of substance. Another aspect to consider is not overburdening the writers.
Schools vary in the number of recommendation letters that they require.
For the schools I chose, I needed seven or eight letters. I found four people to write them, so that no one person was writing more than two letters. While the recommendations are usually very similar between schools, usually each school has a unique form that must be filled out. I wanted to make sure no one was rushing through this process. Also, make it as easy as possible for your recommenders to write their letters.