The best way to keep unwanted guests away, you may have already discovered, is to refrain from supplying them with free meals. In the case of bugs, always store your food (especially sugar and flour) in airtight containers. Clean your counters, tables, and stovetop immediately after cooking and eating. If you're going to leave the dishes until the next morning, at least scrape and rinse them.

Ants
1. Follow the trail and block their entrance with a dab of caulk.

2. If you don't have any handy, squeeze lemon juice into the crack, then cut the lemon and put it around the entrance.

3. Wash surface areas with equal parts vinegar and water.

4. For a final measure: mix 1 cup of flour with 2 cups of borax and sprinkle the mixture around the foundation of your home.

Flies
1. Cover your garbage tightly. When you take out the trash, wash the garbage can thoroughly with soap and water.

2. After it dries, sprinkle dry soap or borax into the can.

3. For a final measure: Make your own fly paper, bring equal parts of sugar, corn syrup, and water to a boil, then spread the mixture onto brown paper, and hang it near problem areas.

Roaches
Never leave food out...not even pet food! If you don't have to worry about children or pets eating toxic substances,

1. Mix 2 tablespoons of flour with 4 tablespoon of borax and 1 tablespoon of cocoa.

2. Put the mixture into shallow dishes, and leave it out for the roach.  

3. If you need a child or pet-safe solution, mix equal parts of baking soda and powdered sugar.

4. Spread the mixture around the infested area. It will kill the roaches.