Every girl should have certain essentials in her purse without becoming a bag lady, weighed down with all sorts of unnecessary stuff. Here's how to find the right balance.

[edit] Steps

Decide what kind of purse you need and buy it. Many smaller pockets will make things easier to find and help control the jumble and mess.
Consider what you need to carry in your purse every day. What items do you use (or find you need) frequently? Make a list of the items.
Consider where you are taking the purse. If it's to go to school or work, stick to everyday items like money, a mirror, breath mints, cell phone, hairbrush, makeup, deodorant etc. If it's to go out, you may want to vary the contents with more makeup.
Evaluate how much makeup you use each day. If you wear makeup every day or go out a lot, you may want to put a makeup bag in your purse. Tissues and a hairbrush are always useful even if you don't wear cosmetics.
If you take medication every day, you may want to put that in your purse as well. If you are prone to headaches or cramps, you may want to keep a small container of pain reliever in your purse.
Gather or purchase the items you want to keep in your purse.
Be sure to include a wallet containing money and an ID. In addition, make sure the purse contains keys for your house and car.
Put everything inside your purse.

[edit] Tips

Inevitably, you will have too many items to fit neatly in your purse, so you will have to take everything out and select something that you can live without.
Clean your purse regularly. Throw out any old receipts and trash. Evaluate everything you carry and make sure it is serving its purpose. If it doesn't earn its keep, don't lug it around.
Make a point to use up your change, otherwise it will all sink to the bottom of your purse weighing you down.
Alternate the cosmetics in your makeup bag. Otherwise, you'll end up toting around ten different kinds of lip gloss.
Keep zippered bags inside your purse to keep smaller items together.
Other items you can include are: mace, a swiss army knife (or Miss A Kit), a book (if your purse is large enough), an mp3 player, a mirror, tweezers, a business card holder, a notebook, a pen, a small flashlight, pads or tampons, hand lotion, a transit pass, an address book, a planner or calendar, a PDA.
For special events, you may want to keep nail polish in your purse to patch up runs in your pantyhose. Or, you could scrunch up a pair of tights or stockings into a ball and keep them in your purse in case of emergency (for example: you have holes in your stockings at an important business meeting).
If you are traveling, it may be a good idea to keep a hand sanitizer like Purell in your purse.
If you are keeping electronic items in your purse, you may want to get protective cases for them. It's also a good idea to "lock" mp3 players and cell phones, so you don't waste power or end up calling your best friends by accident.
If you are "on" or maybe a unexpected mentruation should hit you (especcially if you are a teen and they are not regular) always keep a few pads/tampons in your purse, but be sure to put them in your purse pocket so if you ask someone to hand you somthing out of your purse they don't see it/them.

[edit] Warnings

Don't put unwrapped food in your purse because it will get dirty. Wrapped foods like energy bars are OK.
When traveling on airplanes do NOT bring liquids or gels (unless it's in a quart sized plastic bag).
If you decide to carry mace or a knife, know the laws about these items and get yourself trained in their use.