Find the right spot. To get the optimal angle: Hold your infant upright on your lap, supporting her chin with one hand; drape her over one shoulder; or lay her tummy-down across your lap. Then gently rub or pat her lower back (across from her belly -- not between her shoulder blades).
Take a break to burp. Knowing when she needs to be de-bubbled is mostly a matter of reading her cues. But if the milk isn't flowing smoothly, or if she gets excited when she eats, then she's more likely to swallow air. In that case, burp her when she's finished about half the bottle, or when switching breasts, so that you get the air up before it's under another layer of liquid.
Let her finish first. If taking a break doesn't work, try skipping the intermission. In fact, being interrupted will upset some hungry babies, causing them to swallow even more air.
Don't worry if she doesn't. If your baby hasn't burped after a couple of minutes, she may not need to. Once she's a couple of months old -- spending more time upright and feeding more efficiently -- she'll outgrow the need for your help altogether.