A monitored alarm service (where, if an alarm goes off, the service notifies the police) will knock a percentage off your homeowner's insurance policy, but that's almost all it's good for.
This is because something like 97-99% of alarm service calls are errant, so when they happen, they're the lowest priority thing to do.
In addition, once the alarms go off, the burglar is already gone. The most prevalent crimes are crimes of opportunity: no-one is home, it's midday, they roll up your driveway in a truck, knock, go around back, break in, grab your iPad and PS3 and laptop, take it out the front door, and go.
When they break in, they've got 30-90 seconds before the alarm goes off, 30-90 seconds after that before the service calls, and then 5-20 minutes or more before the cops show up. They're long gone. You don't think someone who's done it before can get into your house, grab a pillowcase off your bed and fill it with anything valuable and portable inside of three minutes? (They totally can.)
If you're worried about someone breaking in while you're home, I can't recommend much, but those are also statistically unlikely. No-one's targeting you, staking out your house, stalking your movements, etc. (Probably.)
If you're worried about someone breaking in while you're gone, you want solid doors, strong door frames, visible security cameras and security lighting outside, and hidden eye-level still image motion-triggered cameras inside, and you want regular inventories of all of your valuables, including proofs of purchase. You want to focus on warding off crimes of convenience, identifying the perpetrators, and restoring what was damaged or lost as quickly and as efficiently as possible so you can get on with your life.
Here's a 2002 US Department of Justice report explaining the problems. Newer reports from local and federal agencies read similarly, in my experience.
http://www.cops.usdoj.gov/pdf/e05021556.pdf
> The vast majority of alarm calls–between 94 and 98 percent–are false (higher in some jurisdictions). In other words, reliability of alarms, which can be measured using these false rates, is generally between 2 and 6 percent. Nationwide, false alarms account for 10 to 25 percent of all calls to police.
Taking photographs is really important. Especially of items like jewellery. They don't have to be amazing hero-shot photographs, they just have to be well lit, clear, in focus, and with some idea of size. (Include a coin, for example.)
If the item has serial numbers include a photo of those too.
This could probably be an app or a website - you give people a checklist of things they need to take photographs of, and how to take good photos, and then let them upload the photos they've taken with descriptions and scans. You store all that in an encrypted blob. (And being careful not to include anything that can lead to the real world address).
> If you're worried about someone breaking in while you're gone, you want solid doors, strong door frames,
Yes. Good doors, and good door frames, are important and they're also nice. I don't know about the US but in the UK you can get insurance rated locks. You'd want to combine a 5 lever mortice lock and a "yale style cylinder" lock.
With a DIY system, I'd always be worried about reliability.
Alternatives: Something slightly odd, but depending on the state Skunks can be a great pet in between a cat and dog. You're not suppose to play rough with them, however I've read it makes people think twice about breaking in. Some other jokes I've read but might help, NRA stickers around the house.
Are you more worried about stray gang bullets, or are you worried about robbery? If it's robbery I'd make sure not to put boxes from new tvs etc outside the house.
Like others said, it's better to well document your personally items, and put in better doors/frames/windows. Which is all very costly.
Edit: Clarify some wording.