David Cameron has come out strongly in favour of gay marriage but has done so in a way that comes across - on first reading - as if he is against it!
Negatives are a minefield when you are presenting because the audience has to unscramble them before they can be understood - we think in positives and pictures. The classic example is giving a small child a tray of drinks with the instruction "Don't drop it"; the way they take in that information is: 'drop it' - got that as the general concept, 'don't'. All the focus is on dropping so that is probably what they do; much better to say "hold it steady" - there is a certain amount they can actually do about that.
So the rule is to change negatives into positives wherever possible, but Cameron's slip was much more fundamental than any of this. He has been widely quoted as saying: "So I don't support gay marriage despite being a Conservative; I support gay marriage because I am a Conservative". It probably helps to be there and hear the inflections and emphasis in all the right places, but when you read the reporting of it what hits you is "I don't support gay marriage......"- which is the opposite of what he intended to say! Aside from sending the audience off in the wrong direction, some wag is going to get a recording of it and cut just where I have.
So what should the Prime Minister have said to get his intended message across? Something like this: "I support gay marriage; some might say I support gay marriage despite being a Conservative; I say I support gay marriage because I am a Conservative." When presenting you want it to be conversational but having made it so, you need to crank up the clarity.