Nod a lot while you talk, especially when leading up to asking for a favor. Scientists have found that when people nod while listening to something, they are more likely to be in agreement with it. They also have discovered that when someone is nodding a lot in front of them, it is natural for them to do the same. This is understandable because humans are well known at mimicking behaviors, especially those that they consider to have positive connotations. So if you want to be extra convincing, nod regularly throughout the conversation. The person you are talking to will find it hard not to nod themselves, and they will start to feel agreeable toward what you are saying, without even knowing it.