Write an article based on this "Link the request with the invite into your house. Ask directly. Inform your guests on the invite. Be firm and polite if they want to refuse."
Try saying, "Do come in — you can put your shoes on the rack." That way the request is linked neatly with the invitation to enter. In fact, it would be very awkward at this point for your guest to do anything other than remove her outer footwear. If your guests remain oblivious to any visual clues you may have provided, you should ask them directly. Be firm, but polite with your request. Most people will respond positively because they won't want to offend.  If you feel it is necessary, give a good reason for asking them such as having had new carpets or having just had them cleaned, the weather being particularly foul, or you can say that your driveway is gritty and you're trying to minimize how much of it gets brought in on shoes. Try saying something like:  “I'd appreciate it if you took your shoes off when you come in. We've just had the carpets cleaned.” “Please take your shoes off when you come in the house. We like to avoid tracking in toxins and bacteria whenever possible.”    Remember that it is your house, so you have every right to ask your guests to remove their shoes. If possible, it is best to let people know in advance that you would like them to remove their shoes. This will keep your guests from being caught off guard in a situation that may make them uncomfortable. It will also give them time to consider bringing their own slippers or wearing socks if they might have otherwise not worn socks. You could say, "Oh, by the way we don't wear shoes in our house. You may want to bring some slippers or socks." Some people may not want to take their shoes off no matter what you say. At this point, you should consider how important your “no shoes” house rules are and decide if you want to cause a fuss about it.  If there is a cultural or religious reason for not wearing shoes in the house, be open and let your guests know how important it is. For example, “I'm very sorry this caught you off-guard, but where I'm from taking your shoes off before entering is a sign of respect. It would mean a great deal to me and my family if you could do us the honor of entering our home without your shoes.” Remember that you are well within your rights to ask your guests to remove their shoes, but if this makes your guests uncomfortable, they may choose to leave. That is a risk you must be willing to take.