[edit] Steps

Send out invites at least a week in advance. Call invitees, email them, or ask them personally. Remind them to bring a sleeping bag, pillow, etc. Unless it's a birthday party you don't have to make invitations. Limit the amount of girls you invite if you don't want things getting crazy.
Stock up on junk food and goodies. Caffeinated drink and sugary foods are good for staying up all night. Good snack foods include popcorn, pretzels, cookies, ice cream, etc. Ask guests beforehand if they have any food allergies however.
Rent movies or pick out some good movies from your collection. Ask around before the party to see if anyone wants to see a certain movie. Romantic comedies are usually a safe bet for a bunch of girls.
Plan some fun slumber party games:
- "Pick a Question": Everyone writes down a few questions, such as "Who will be the first to get married?". Then everyone answers them on a note pad.
- "Would You Rather": "Would you rather kiss a monkey or wipe an elephants bum?" So childish but funny at the same time because you have to answer.
- Play truth or dare.
- Play Charades or Sardines, which are great group games.
- Board games and card games are another option. Ask people to bring their favorites.
Tell secrets.
Tell ghost Stories.
Get makeover supplies. Make face masks, paint fingernails and toenails, or apply makeup, but don't make a mess.
Ask everyone to bring in their favorite CD's so you can play them on your stereo during the party. Or you can just turn on a radio station you all love.
Prepare the house. Decide where you want everyone to put their stuff and hang out for the majority of the time. Place a trash bin in the room so you don't end up cleaning up everyones candy wrappers.
Have the party! In the end you'll probably still have a blast even if you're not completely prepared activity-wise. Just have fun.
Order food. Take out food, like pizza or Chinese food, is a great idea.
Use a camera to take pictures of the party. It can be fun posing and sharing the photos afterwards.
Make or buy breakfast. You can ask your parents to make waffles or pancakes or have them buy doughnuts and pastries from a bakery.
Help everyone pack up and thank them for coming as they leave.

[edit] Tips

Turn out the lights when people start trying to go to sleep. People will start drifting off to sleep when the lights are out anyways. Others can always whisper quietly in another room.

[edit] Warnings

Don't be loud. It'll annoy parents and neighbors if you're rowdy enough. Go into a room and close the door if you tend to get a little loud.
Don't make too much of a mess. It'll be a hassle when to clean when they leave the next day.
All secrets told at the sleepover stay at the sleepover.
If your telling ghost storys don't do it right before you go to sleep . Same with horror films. Maybe watch a comedy after. (Once I had a sleepover where we told ghost stories right before we were ready to sleep.. My friend got scared and went home :-( )