Deep throating is something that is often mentioned in erotic stories, and often seen in pornos and on the internet, but most girls (and guys) find it rather difficult to do. Part of it is a sensitive gag reflex, part of it is the fear (and the reality) of not being able to breath with a dick shoved down your throat. However, it is possible, and although I'm not a guy (Not since I last checked anyway) I've been informed by my very male fiancΓ© that it's one of his favorite things during sex (admittedly, on a rather long list of "favorite" things).
The first thing you can do is practice, not necessarily with a banana (although I know that some people do use that successfully, I don't really think it's actually necessary), you can practice just with your finger or a toothbrush. Just every time you brush your teeth, at the very end brush your tongue as far back as you can get it, until your gag reflex starts screaming at you. The constant pushing on it will actually make your gag reflex less sensitive, and allow you to get objects further and further back into your throat. A good reason to use your finger is that most guys are not as long or as hard as a toothbrush is, and with your finger you can get a good general idea of where the most sensitive spots in your throat are. Knowing the "danger" zones in your own mouth is a HUGE bonus when it comes to attempting to swallow a dick.
Anther pointer is to do just that, swallow while sucking, making the swallowing movement with your throat and tongue. Your gag reflex tends to relax when you swallow, allowing you to move the tip of the dick past it, for some reason the gag reflex doesn't seem to have a problem with a dick laying under it, just poking into it. However, eventually you will need to pull off and repeat this procedure so that you can breathe. Breathing is important, if you aren't getting enough air you're actually more likely to gag or actually throw-up, also if you're trying to swallow the dick and gasping for breath at the same time because you haven't gotten enough air, chances are you're going to swallow air, which is again, going to upset your stomach and make you gag or throw up. One which isn't very pleasant for you and one which isn't very pleasant for you or your partner.