In case I didn't make it amply clear in this post, there's one person I blame if a woman's in-laws aren't nice to her: her husband. So if you're an (Indian) husband, I hope this post will help you ensure that your wife isn't unhappy because of how your parents treat her.
First, make sure you know how she's being treated. Not just when you're present, but also when you're not. Listen for clues: how do they speak of her when she's not there? How do they speak of her to neighbours/friends/other relatives? Do they show consideration for her likes and dislikes when you're invited for a meal? Do they ask about her day, or just about yours? Do they talk to her as a person, or just as a part of the couple the two of you form? Do they refrain from commenting on her fashion or lifestyle choices (whether she wears short skirts, or drinks, or works late) even though they might not agree with them? But just listening is not enough: ask your wife what she thinks of your parents and how they treat her. Don't wait for her to seethe and hurt and finally tell you when the dam bursts.
Second, when they say anything in the least disparaging or rude, stand up for her at once. For instance, if they say, "If only you would wear a sari..." say, "Oh no, saris are so uncomfortable! I think she looks lovely in jeans!" If they say, "It's sad that you have to work such long hours, my dear," you say, "Why? She's successful and she enjoys her work. I'm so proud of her!" Make it clear that you approve of her choices, that you support her lifestyle and her decisions, and that you will not brook criticism of her.
Third, block their attempts at interfering in your life. Suppose they say: "I know Asha thinks it's too soon to try for a baby, but..." You say: "Asha and I have talked over it and we agree." "I wish you would move back to our city." You say: "Asha and I agree living in this city is better for our careers. You know you're always welcome to visit."
Fourth, make sure you always treat your wife with respect. If you raise your voice at her, you are sending the message that it's okay to treat her that way. If you don't listen when she's speaking or greeting her with a smile (at least!) when she comes home, you're belittling her presence in your life. Be extra careful about this for the first year or so (after which I hope it'll come naturally), when your parents are trying to know her and gauge your relationship. Once they accept how much you respect her, they are much more likely to respect her too.
The bottomline: always make it clear that your wife is the most important person in your life. Your parents should realise that insulting/hurting her will result in you liking them less. If they continue to disrespect her you should shut them out of your life: but if you make it clear from the beginning that you'll go that far if need be, you may never need to.