Stick up window stickers that warn of an alarm system or a dog. These can act as a deterrent.
Purchase a do-it-yourself alarm kit. Many big-box stores sell security cameras and other easy-to-install, relatively low-priced theft deterrents that you install yourself.
Install a steel jam for your sliding glass doors or sliding windows. (Many local home-improvement stores sell complete door-jam kits.) The jam prevents the sliders from moving at all. You can also use wooden dowels that are at least 1 inch thick.
Install security door bars that go right under the door knob all the way to the floor (this can only be done while you're inside). You can buy one for under $20 at a home improvement store.
Change your routine (as much as possible without disrupting your life too much). If you have been coming and going the same time every day, leaving in the same car, try to mix it up a bit. Leave or come back at a different time some days. Carpool every now and then. Leaving a car in the driveway can deter potential burglars by making them think someone is home.
Install timers on your lights so that it looks like someone is home if you're on vacation or working extra late. You can program these lights to go on and off at specific times.