Choose an open aperture (or low f-stop number) between f/1.8 and f/5.6. The lower the f-stop number, the less depth of field (or how much of your photo is in focus). This is the #1 way to get nice bokeh in the background.
Choose longer focal lengths (50mm or higher). Longer focal lengths allow you less depth of field. The less depth of field you have, the more background blur will be present in your photos.
Keep your background far behind your audience. The greater the distance your subject is from the background, the more out of focus it will be. Also, the closer you are to your subject, the more out of focus the background will be.