I bet for a living but learned not to get too involved in betting on Strictly when, in the series 6 semi-final in 2008, the BBC changed the rules without warning and put the three semi-finalists through to the final which Tom Chambers duly won. Chambers was a cert to be eliminated at the semi-final stage after the judges had scored the dancers (barring an absolute disaster in the dance-off by Rachel or Lisa) and thus I took him on to win the tournament. Fingers were burned!
There have been opportunities to cash in during other series, though. When Zoe Ball was eliminated in third place in series 3 the money went down immediately on Darren Gough and was rewarded. There was only a very brief window to get on before others realised what was happening and his price was slashed. The perceived wisdom that Erin's decision to choose "dancing dummies" cost Colin Jackson the title is not correct.
Generally speaking, it often pays to lay the pre-series favourite on shows such as Striclty, Dancing on Ice, the X factor etc as if you back these before the show starts its run you are doing so not on the basis of any performances but on speculation. Things will happen during the series. Dramatic improvers emerge and can go on to win. Even the favourite may have a bad week and give you the chance to get out of the bet and minimise the loss if you have taken them on.
That said, there are always exceptions and this year I wasn't going to lay Natalie as I thought with what we knew about her she really would be the front runner from day 1 and the usual opportunities to trade out were unlikely to emerge. Indications are that this will be the case, whether she wins the title or not.
It is also worth bearing in mind that you are betting against people with inside knowledge of how the public vote has been shaping up each week!