mel,
I find that with tetracycline it can take a few days, or even after the end of the (usual) 5 day course before the effects are noticed. It takes that amount of time to start killing off your normal gut flora and the potential overtake of bugs that can cause the d*. As nikki said, yoghurt is good as it can help replace the invading bad bugs with a nice healthy flora of good ones (its a battle in there I can tell you). Also helpful are the capsules containg large doses of freeze dried 'probiotics' - look in your local health food shop for them. I can't remember brand names and they are probably different over yourside of the pond anyway.
As for those who take minocycline, it is not the nicest of antibiotics. I was on it for about 2 years (for severe acne) and I felt really ill on it. A lot of research was published about 6 or 7 years ago saying that minocycline can have very bad effects of the kidneys and a lot of doctors were withdrawing it.
Perhaps the best treatment for acne, although probably the most dangerous is Roaccutane, which should be administered with constant medical supervision due to the serious liver side effects, however it is very very good at its job and I have seen it clear very painful bleeding and cystic acne in 3 months and for it not to return.
Hope this helps,
Iain