Nodding
The simple trick of nodding your head as you speak can go a long way when you are trying to get someone to agree with you. While in the middle of a conversation, just try nodding a few times and you may come off as very deep and thoughtful to the other person.
Make sure that you do not do this too much, it could look like you are bobbing your head and if you nod too fast, it looks aggressive and just strange. The slightest of nods is a little tool in your pocket of tricks that can make you more agreeable to anyone.