Three cheers!
We are in the same boat with our 18mo. We also just intro'd time out.
It felt very weird the first time, as I'm sure little man had no idea what was going on - but it only took about 3 or 4 time outs before we could mention it and he auto-corrects.
We use it wherever - at a golf outting this weekend I just sat him down on the gravel about 10 feet away from everyone.
We tend to talk through the time out, calmly explaining that swinging upside down from the chandelier can hurt little boys, and daddy told you to get down, but you did it again and that's why you're in time out.
Sometimes he cries and tries to walk away - actually that's only happened twice now that I mention it - and I physically held his hips in the chair.
We do wait until he's calm, which after the introductory time outs, really hasn't been an issue. Oh, and we really don't time it unless he outright defied us (looked us in the eye, sneered and did forbidden action). Neither of us wear a watch, so we range anywhere from 15 seconds to a full minute.
I haven't seen a behavior "extinguished" - but I have seen him now take a glance at me before he engages in it again and a raised eyebrow now does the trick. Well...shoot, again now that I mention it after about 3 timeouts for playing with the electrical cord for his brother's swing, he hasn't touched it in a few weeks.
I think that if we do it right (consistent, swift and certain discipline) - then we're really only dealing with the normal toddler trying every so often to push boundaries - and that's easy peasy compared to other toddlers I've seen (family).
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Just read some of the responses mentioning NOT holding him down. I took that from some of the other moms on MP from a question about tantrums when changing a diaper. Quite a few of the moms mentioned changing a diaper solo (as I imagine we all do) and they didn't hesitate to put child on floor and throw a leg over child to hold the upper body down.
Not hard or kneeling on child, but enough to get the diaper changed. So that's where *I* felt it was fine to hold his hips down in those first few time-outs so that he knew he could not get up. Only did it twice, and haven't had an issue since.
Also, when we first started time-outs (just a few weeks ago), it seems like we were over-doing it (4 to 6 per day). I think it was more of him finding his boundaries as now we only have a time out maybe once every couple days.