Communication, communication, communication!
First and foremost is finding the right set of speakers. You need to find a group of presenters whose work complements one another, but not with so much overlap that every one is bored of nearly identical introductions by the end of the session. I suggest inviting people you have personally heard speak and who you think give very dynamic presentations...some people have fantastic work, but give horrid presentations, or have a well-known habit of running overtime (if you can't avoid having one of those, schedule that person last so they don't run into everyone else's time).
Other things that are important are keeping in touch with the presenters in advance, making sure they know the other people who will present and the titles of their talks as soon as possible so they can tailor their presentations based on those that will come before and after theirs. Confirm a couple months, a month and two weeks in advance that they are still planning to attend, and make sure they know how to reach you if they have a last minute emergency that keeps them from attending.
Make sure you know what audiovisual equipment will be available well in advance, including if there are restrictions on what operating system or presentation software will be available, or if people can bring their own laptops, and convey this to the presenters so they can check their presentations are compatible in advance.
Plan to arrive an hour early to meet your presenters and get their talks loaded onto the computers so you have time to fix any technical problems before the session begins.
It's also nice if you can get a trainee as a co-chair so they can get some experience in chairing sessions too, and let them introduce a few of the speakers.
That's the biggest part of it...select the speakers, get them there, introduce them, keep them within the time limits, and since you'll have the most advance knowledge of the content of their talks, be sure to have a question or two planned ahead for each one just in case the audience is slow to ask questions (the session chair is somewhat obligated to ask a question if nobody else does so that the presenter doesn't leave feeling like nobody appreciated their talk).
Oh, the other thing that helps is to make sure questions are taken from people in the audience other than the ones everyone already knows by name. If a young person who might be a student timidly raises a hand, give them a chance to ask a question before they lose their nerve. I hate it when you attend a talk and have a question, but only the "in crowd" gets recognized, especially when you're really trying to stretch and learn something more outside your field.
Oh, and if you have any say over it, think about bladder capacity after consuming coffee when planning shorter breaks...have an break earlier in the session in the morning than you might plan in the afternoon, or else make sure the room is arranged so people can easily slip out without having to walk past the speaker.
That's the basics. The reality is that most people complain if 1) you get horrible speakers presenting only stuff they've already published rather than adding in something really new, 2) the AV equipment is malfunctioning and distracting everyone or making it impossible to hear speakers or see their slides, 3) you let the speakers run over time so that the session becomes unbearably long.
If you have more than just one session to plan, the logistics get a lot harder (I've been on the local arrangements committees for a few conferences...one for a large, national meeting with multiple concurrent sessions held in a convention center and two different hotels...so if you're actually planning an entire workshop and not just a single session in one, I can cover some of the other logistics, including don't hold concurrent sessions in more than one building if you only have one person to run between them to fix AV problems

, but that gets to be a pretty long list.)