When it comes to asking your parents for something you want, timing is everything. Your mom or dad are much more likely to say yes in the middle of a relaxing Sunday afternoon on the porch than when they come home from a stressful day at work or have spent all day cleaning the kitchen. Look for a time when your parents are relaxed, when they don't have a million things on their to-do list, and when they're generally happy with you. Though it may be impossible to know exactly what's going on in their minds, you can watch them for a little while to get a sense of whether or not they would really listen to your proposition. Though you may not know exactly what your parents are worrying about, you should avoid asking them right before your grandparents visit, when they're doing work at the kitchen table, or when they just generally seem too tired to really listen. Though you can't wait forever to ask them, it's better to wait until the time is right unless you're really in a rush. Another thing to think about is when you actually want to have your sleepover. Don't ask to have the sleepover the night before your grandma comes to visit, and don't ask to do it when you know your parents will be doing some spring cleaning. Pick a time when they won't have much to do around the house or too much on their minds. The more convenient the time, the more likely they'll be to say yes. You should also pick a time when you are relatively free so they'll be less likely to make an excuse like, "You have a math test/soccer game/spelling bee competition that day." You may really, really want to have a sleepover as soon as you ask for it, but it's important to think at least a few weeks ahead to improve your chances. When you approach your parents to ask for something, whether it's for them to buy you a new video game or to get a ride to the movies, tone is everything. If you come in there acting like, "I'm going to have this sleepover and you're nuts if you think you can stop me," then they will say no faster than you can walk through the door. Instead, be kind, understanding, and let them know that they have the power. Instead of rude statements, use a polite statement like “[parent name here], I want to have a sleepover with my friend [friend name here]. May you allow it please?”. This will make them much more inclined to say yes. Though it can be hard to remember to respect your parents when you really want them to give you what you want, this is something you should do at all times. After all, they make the rules around your home, and provided that they are fairly reasonable, you should avoid complaining or being mean if you don't get what you want because it'll only make things worse. If you've never had a sleepover before, it's probably not a good idea to invite every girl in your fifth grade class to spend the night at a Twilight themed party. Instead, just invite two or three friends, at most, and don't have any special theme or complicated food requests. If your parents let you have a basic fun and simple sleepover, then they may be willing to let you do more in the future, but you should avoid overwhelming them with complicated requests because they'll be more likely to say no. You may just expect your parents to say yes and give you exactly what you want. However, you should think more about it and realize that they would appreciate, and that they would even deserve, to have something in exchange. You can offer to help do the dishes more, to do more of the chores that your parents do, or to find another way to help out either around the house or in some other way. If you're old enough, maybe you can help with the grocery shopping, walk the dog, or do something else to give your parents a bit more free time.  You shouldn't even wait for them to say no. When you finish asking your question, you can finish by saying, "And in exchange, I'd be happy to clean out the fridge/take out the trash all month/clean the kitty's litter box from now on." Think about it: is there something your parents really dread doing that they would love for you to do? Maybe they always complain about picking up the mail, answering the phone when they know a telemarketer is calling, or having to weed the garden. See if there's something that you can offer to do that they would love so much that they'd have to let you have a sleepover in exchange. Though you don't want to make your parents feel guilty about making you look like you have no friends or not letting you hang out with people, you can make a casual point about how it's pretty common for kids your age to have sleepovers and that you don't want to miss out. Tell them that you'd like to hang out with your friends in a nighttime setting and that you think it'll be a fun way to get to know them better. Make your parents feel like they'd be making you miss out on a fun opportunity. Okay, so if you've tried being nice, pointing to times when you were responsible, or just saying "pretty pretty please?" and it's a no-go, you can try some tactics that are a bit more drastic. Here are some options:  Tell your parents that your friend really wants you to sleep over her house. After they say no, you can say, "Well then, can she sleep over here?" Most parents are really afraid of having their kids go sleep in some other parents' home and are much more comfortable letting kids sleep over their place. If this is the case with your parents, then they'll be more likely to say yes after you pose the first question because they'll see having the sleepover at your home as the lesser of two evils. Ask them if you can do something more extreme first. You can also ask your parents if you can do something a little more extreme, like going on vacation with a friend and her family, getting a dog, or taking horseback riding lessons, and then wait for them to say no. After they've said no, be really upset and give it a day or two so they think you really mean it. Then, casually ask them if you can have a sleepover. If they're not on to your tricks, then they'll be more likely to say yes. If you have a pesky younger sibling or two, you can say that they can also join the sleepover. That way, your parents get a babysitter for free and can take the night off.

Summary:
Ask your parents when they are in a good mood. Ask to have the sleepover at a convenient time. Be polite instead of demanding. Keep the first sleepover simple. Offer to do something kind in exchange. Show them that it will be a good opportunity for you to socialize. If the basics aren't working, try a more drastic approach.