How to tell professors I am not interested in working with them right now, but might want to, later? I am PhD student who was (and still am) trying to find an advisor. Over the summer I contacted many professors, and started small 'trial' projects with them. Since people respond at different times and move at different paces, all of these projects started a few weeks apart, towards the end of the summer. I very recently decided I want to focus on one project with the professor I liked best and want to politely let the other professors know that I no longer want to work with them on their projects. I had not met these other people more than thrice (in three weeks), and had only got to the point of finishing reading a couple of their papers. None of these professors was/is paying me for research, either. 

Here's the thing, though - the professors who I don't want to work with are the ones who are willing to take me right away, because I have the skill set they want in their groups. The professor who I want to work with wants to 'test' me for a quarter, so there's always the chance that he might eventually reject me. I however do want to give it a fair shot, since I loved his work, working style and personality the most (we've met twice so far, and both were technical meetings). 

So coming to the point, I want to have the other professors that like me as back-ups in case this particular professor is not sufficiently impressed with me by the end of the quarter. How do I convey that to them without pissing them off?
You probably know your professors better than anyone on here, as you have interacted with them. Different people respond to rejection in different ways, and this can be a delicate matter.

However, you did note that the professors that you don't want to work with were all responsive to you, which may indicate that they are invested in students. In my experience, faculty who are invested in students want them to do well, and understand that students' research interests will not always perfectly mirror their research interests. Getting a Ph.D. is a long, difficult process, and your professors have gone through it themselves! Thus, they likely know the importance of selecting a research area/advisor that will work best for the student.

My personal advice: Be honest with the professors that you don't want to work with. Tell them that, while you are interested in their research topics/methods, you found a faculty member whose research interests are highly congruent with your personal interests and long-term career goals. You can also tell them that you feel scared that the other faculty member won't take you on, which could open the door for you to directly ask them if they are agreeable to you coming back to work with them, should your relationship with your first choice advisor not work out. Express your appreciation for their responsiveness and for them giving you a chance to work with them.

Some faculty may respond negatively to you, and they may view you as being unreliable or untrustworthy, since you are leaving the projects. I would hope that your faculty will be understanding, if you are able to be open about the situation.

Best of luck!