The prospect of informing your boyfriend that you want more out of the relationship is a sensitive one. On the one hand, you don't want him to get defensive and turn things around on you by saying that you're too demanding. On the other, you deserve to be able to communicate and get what you need out of the relationship. Fortunately, the process is not as hard as it seems if you employ an honest, fair and direct communication style.
Establish your independence beforehand. The less needy you appear to be, the more likely he will be receptive to your request. Men appreciate a strong and independent woman who has her own life and isn't codependent, which is why your wanting more won't seem laden with ulterior motives, i.e. "NOW what does she want?"
Decide on what it is that you specifically need more of. The more vague you are, the more likely the conversation will turn tense and lead to a dead end. If anything, you don't want to take a step backwards here. For example, if you feel that he is dragging his feet about becoming exclusive, ask him what he's looking for first. Then, politely yet firmly state what you need, and suggest working together on how to get there.
Set up the time to talk in a tactful manner. Schedule a relaxing day date such as picnicking in the park, where conversation will come easily and you two will have ample privacy. Avoid situations that pose time constraints like dinner at a restaurant in the evening or bringing things up right before one of you has to be somewhere. Lastly, stay away from the dreaded "we need to talk" phrasing when setting up the time, as this could shut him out emotionally before the conversation even begins.
Use honesty and straightforwardness when telling him that you want more. Even if he's temporarily taken aback by what you're telling him, he will appreciate you not beating around the bush. Don't overcompensate by being critical, however.