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I am using a passive infrared sensor TMP006 to make a burglar alarm. However I notice that the data returned is not accurate enough. Are there any other ways by which I can improve my accuracy. Or should I use an ldr on which I shine a continous beam of IR light which breaks when the burglar steps in?What do commercial burglar alarms use to detect motion?

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The TMP006 is a PIR, but a temperature sensor. This should be used in a situation where the temperature to be measured is equal over the full viewing angle. That would require the burglar to be very close to the sensor, something he might not want to. LDRs are no use either, they detect light levels.

The common way to detect a person's presence is a PIR presence/motion detector. I've used the Panasonic NaPiOn for this, which at the time was the smallest existing PIR.

enter image description here

It has a matrix of detection zones, which, combined with the multi-faceted lens will detect even slight movements at meters distance.

Use is easy. It has three pins: ground, Vcc, and output. The output goes high if the sensor detects a person moving in its detection field.

Further reading:
NaPiOn design manual

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TMP006 is an pir. I have mentioned it in the question(google.co.in/…). I will definitely take a look at the panasonic NaPion. It seems interesting – Sab Jul 18 '12 at 6:42
@Sab - Correct, it's a PIR, but a temperature sensor, not a presence/motion detector. – stevenvh Jul 18 '12 at 6:48
Okay. That makes a lot of sense. Thanks for the help. You said you have used NaPiOn. Did you interface it with a specific board? – Sab Jul 18 '12 at 7:06
@Sab - Thanks for the accept. I used it professionally, on a self-designed microcontroller board. I don't recall the microcontroller type, but that's not important; it will interface with about any microcontroller. Power it with the same voltage as the controller (it works on 3.3 V and 5 V), and connect the output to one of the controller's inputs. The design manual shows a pull-down resistor, but IIRC I didn't even use that. – stevenvh Jul 18 '12 at 7:13
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@ppumkin - thanks for the link. Like I said to Sab these aren't cheap, but I needed them for their small size. Other PIRs may be less expansive. – stevenvh Jul 18 '12 at 11:12
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AFAIK, LDR is not suitable for motion detection. I'm using PIR Motion Detector, works quite well.

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Security system motion detectors use the detection mode of the combination of infrared and microwave, i.e. well-known dual technology. The microwave detection mode of the dual-technology detector is as follows: According to the Doppler Principle in physics, microwave sends at a frequency; when there is any moving object in its coverage range, microwave reflects at another frequency. Therefore, there is a frequency difference between the sending frequency and the reflection frequency. One's volume and moving speed can be gotten according to the detected variation. In combination with the infrared detected signals, the detector then performs operation processing of both triggering signals through the internal CPU chip, comprehensively analyzes relevant characteristics of the signals, and finally judges whether there is any invader in the visual area.

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Specifically, the microwave sensor is sensitive to motion toward or away from the detector, while the PIR detector is sensitive to motion across the field of view. Together, you get detection of motion in any (horizontal) direction. – Dave Tweed Oct 3 '12 at 15:52

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