Form: Generally overall, it should be loose and fluid, tensing up with a good snap 2-3 at the end that's targeting a spot 2-3 inches behind the target. It should return high and recoil to a position where it can be fired again, allowing for rapid fire deployment. Your weight will be distributed evenly but shift to the ball of your fore foot as you're extending toward the snap.(I used to use a sheet on a clothesline when practicing my jabs when solo training. If you hit it dead center and get a good snap, you'll hear it and the sheet will pop and indent jumping up from the bottom; if you're pushing it instead, you'll notice the lack of movement in the sheet.

Some fighters step into their jabs to give it a little extra oomph (over just shifting weight) on it like a power jab. Its effective for keeping especially heavier opponents off balance. If you have an opponent that likes to parry you have to beware of over committing yourself with it as that step as it widens your stance and brings you further in range.

When jabbing to the body, you should be dipping at the waist and knees to avoid keeping your shoulders high, jabbing low and exposing yourself. Also you'll make the most of your reach horizontally.

Its all about results really. That will tell you if you're using it effectively. Due to differences in reach, height, and technique your opponent can negate your jab rather well.

Defensively: It should keep keep your opponent off balance, keep him/her for setting their feet and give you a good idea of range and line of sight (i.e. to avoid being a sitting duck for the overhand right if hes an ortho--you either want to be to the left or the right of that center line.. if your dead center and can touch them with the jab, they can go for broke with that right hand).

Offensively: it should set up your other punches, distract your opponent and keep them honest. You can hope to open them up, but some fighters just cut/ swell easy and other don't.

If your opponent is stuffing your jab effectively, it can still be used for feinting. Defensively, it can cause that second of hesitation, allowing you to use your footwork to get you the hell out of dodge. Offensively, if you have an opponent that likes to parry feinting a jab motion from the shoulder can get their hand high (pulling that elbow away from their side) momentarily.

Sorry for the long post but this is really the abridged version... volumes can be written on just the jab...