Refusing Permission for a Search
You may not know this, but you're within your rights to withhold your consent for a search. As long as you remain calm, and make a polite, clear and firm refusal, in many cases you can keep the police out of your home. If they decide to enter anyway, what they find may become inadmissible in court, as they obtained it in an unconstitutional way, without your consent.
What's more, even if you initially do give your consent for a search, you can reverse your decision once the search gets underway. You have to make your refusal explicit (for instance, saying "I want this search to end now"). However, keep in mind that if the police have already found something suspicious, they might be able to ignore you at that point and keep searching.
That's why you should try not to give them consent to begin with. Even if you're innocent of a crime, you have no idea what the police could find and what they might hold against you once they're in your home. In the course of their search, the police might also cause damage to your home and make a mess; furthermore, searches can take up a lot of time.
To protect yourself, it's best to calmly and politely turn down a request for a search. The police aren't obligated to remind you that you have this right. You need to know it for yourself.
Refusing Permission Has Its Limits
In some situations, police can enter your home without a warrant and without your consent. For example, if there's an emergency or if they spot something illegal in your home from the outside, they can go in. Also, if you get arrested at home, they can look around for additional evidence.
Regardless of the situation, stay calm and remember to contact a reputable criminal lawyer. Sometimes, evidence becomes inadmissible because the police collected it in an unconstitutional way. You need a lawyer to safeguard your rights and protect you if the police overstep their bounds.