Understanding the basic rules on treating stains and spots will help you deal with them more effectively. Below are some general tips that apply to *most* spots and stains, but the first two points are essential in treating the whole gamut of stains that you may encounter in your apartment.
Jump into action: The longer a stain has to set, the more likely it will become a permanent part of your home decor. The best time to treat a stain is within minutes of its occurrence.
Know what you're dealing with: Not only is it important to know what caused the offending stain, but it's equally as important to know what the material is that's being stained. Both tidbits are essential to correctly treating a stain, as one material is not treated in the same way as others. Choosing the proper stain removal technique for the materials at hand is key.
Remove as much of the cause of the stain as possible: The less mustard you have to deal with the better, so scrape/soak up as much as possible. Use a paper towel to absorb excess liquid (don't rub, just place a clean corner of the towel in the liquid and let it absorb it). For solids, rub off the excess with a dull knife or spatula. Powders should be shaken or brushed off. Try not to spread the stain when removing the excess.
Be gentle: The more forcefully an item is handled, the more likely the stain will penetrate deeply and damage delicate fabrics.
The heat is OFF: Don't use hot water on any stains, nor dry them with heat. Heat is the perfect source for making a stain permanent. (An exception to this is wax, as heat is a good source for removing excess wax.)
There will be a test afterwards: Be sure to test a stain-removal solution, as even water can be damaging to certain fabrics. So conduct a test in an inconspicuous spot (near the seam of a garment, or under a table or chair on a rug/carpet), to avoid costly mistakes.
Read the fine print: Follow the manufacturer's directions to the letter if you're using a commercial stain removing solution. If you make your own cleaning supplies, be sure to use the proper ingredients for the fabric, and use the solution exactly as described.
Meet in the middle: Start cleaning from the center of the stain outward, as such movements help avoid leaving a ring around the cleaned area.