I completely disagree with ICAL Pete. As a former trainer of a CELTA-alike course, I (and the other trainers) found that people who had taught for a couple of years before CELTA were into such bad habits that their first two weeks were entirely wasted getting out of them. Add to that the fact that they had spent two years in crappy jobs teaching badly, all unnecessarily. Once you have the CELTA, you can turn yourself into a great teacher for free by reading, attending workshops etc, all things that you would get much less of without that initial training. Also, CELTA might be intensive and nerve wracking, but it is nothing compared to four weeks of teaching without any training at all.