In the midst of the no-knock-raid controversy, several people have claimed that there is no other way to get the evidence in a drug search warrant because the drugs will be flushed. But I doubt that; there must be other tactics that could do the job without terrorizing people.
Now look, I'm not a policeman or a lawyer; I'm just an engineer, but changing people's behavior is not that hard when you have as much power as the government does. It's just a matter of setting up incentives and penalties, making it harder for people to make the decision that you don't want them to make. In this case, you want to make it harder for a drug dealer to decide to flush the evidence.
Well for starters, pass a law that makes it a more serious crime to destroy evidence that has been the subject of a search warrant. Or failing that, at least the prosecutor could announce a policy of not plea bargaining on the destruction-of-evidence charges. But don't just increase the penalty; also increase the risk of paying the penalty. Police investigators should start using a plumber after a raid where they think evidence has been flushed to recover the evidence and to help prove that evidence was destroyed.
Make sure that drug dealers know about the new penalties and about the new tacitcs, so they have a more complicated decision about whether to flush the drugs: "maybe I'll get off, or maybe things will get a lot worse." You wouldn't have to use a plumber every time, just often enough to make the dealers afraid of the possibility.
In addition, the police should start doing warrantless search attempts. When the police are suspicious that there are drugs in a house but they don't have enough evidence for a warrant (or they do have enough evidence but don't want to bother), they go to the house just like they were serving a knock-and-announce warrant, clearly announce themselves as police so everyone in the house knows it, and then ask for permission to search the premises. Some criminals are dumb enough to allow the search and some of the others were smart enough to flush the drugs as soon as the police knocked on the door. So even if you don't send the dealer to jail, you may get the drugs off the street and cost the dealer money.
The warrantless search attempts should be very convincing so that the residents don't know whether it is real or not until the very last second. If the police want to burst in after everyone in the house knows that they are police, that is a lot less objectionable.
There should be about three or four warrantless raids for every real raid because this makes the question of whether to flush the drugs even more complicated for the dealer: the police knock on his door and he's thinking, "The last three times the police knocked on my door I flushed the drugs and they didn't even have a warrant. Now I'm in serious debt to my supplier and can't get away with flushing any more drugs because my supplier will do something a lot worse than sending me to jail. And if it is a warrant and I flush the evidence, it could cost me a lot of extra jail time."
If there are two drug dealers in the house then one of them can stand over the toilet while the other answers the door, but that doesn't work out because the police won't show the warrant until they have clearly announced themselves and been recognized. Then they might just barge in and catch the dealer in the bathroom destroying evidence and they would have a conspiracy to destroy evidence as well.