Just as a locked door can keep intruders out, a properly sealed door can help keep out unwanted pests. If you examine your entrance ways carefully, you'll likely be surprised by the number of unsealed gaps you see, each of which makes an easy entrance path for bugs.
To keep insects from crawling under your door, install a sturdy steel or aluminum threshold under the door. For even better protection, combine this with a door sweep. A sweep is a cheap device that can be found at most hardware stores, and helps to cover the gap between a threshold and the door bottom. Choose nylon brush sweeps over vinyl or neoprene, as they offer the best protection against bugs.
Add weatherstripping or door-seal kits to the perimeter of the frame to keep insects from entering along the sides or top of the door. Use a clear caulk to seal the joint where the door frame meets the wall and also along the bottom of the frame, at the area where it meets the ground.
Remember, all of these door sealing techniques won't help if the door is left open. If you have forgetful kids, consider installing a door closer. This hydraulic device will automatically close and latch your door after it's been opened and can be adjusted to control closing speed and power. You can find affordable door closers at your local home improvement store, and the average homeowner can install this device using only simple tools.