There is no precautionary measure as such acts but you can definitely file a police complaint in the nearest police station regarding the blackmailing and the false accusations. In case the police station refuses to file the complaint, you can approach the Superintendent of police with your complaint. You can directly approach a Magistrate under 156(3) of CrpC if you are neglected by the police .
Keep evidences of all the circumstances where you can record the conversation between you and her. You should definitely collect evidence to prove that you have never demanded dowry from her or her family. Evidence of this kind will help you to get anticipatory bail under Section 438 of CrPC. FIR is not a precedent for acquiring an anticipatory bail. Likelihood of arrest based on a reasonable belief should be shown for getting an anticipatory bail .
In case your wife does file a false accusation, you can file a complaint against your wife for instituting criminal proceedings against you for which she had no lawful grounds under section 211 of the Indian Penal Code.
You can also bring action against her in civil court for malicious prosecution .