There are lots of techniques for becoming more persuasive, but perhaps the simplest and most practical technique is the "But You Are Free" method. A review of 42 psychology studies involving 22,000 people suggests this technique could double the chances someone would say "yes" to you. Read on to see how this works, but only if you want to.
See what I did there? That's basically the "But You Are Free" technique: make a request, but acknowledge the other person has a choice. PsyBlog explains that this persuasion technique reaffirms the person's freedom of choice and indirectly tells the other person that you're not threatening his/her right to say no.
The actual words you use don't seem to matter; according to the meta-analysis published this year by Christopher Carpenter in Communication Studies. You could add to your request "but obviously do not feel obliged" or "but you are free", and both would be effective — especially if youare asking something of someone face to face.
Obviously, you wouldn't want to use this technique all the time, lest you start looking like someone with passive-aggressive issues ("Hey, honey, can you take out the garbage? But you are free not to."), but acknowledging the other person's ability to choose could make that person feel more empowered. It even works on stubborn children and adults sometimes ("I like this choice best. But you're free to choose another one.")
Whether you decide to incorporate this technique into your persuasive arsenal is, of course, up to you.