Here is my thoughts on ab training and the exercises I can think of, off the top of my head.

Strong abs are essential to a solid and strong body and they play a vital role in supporting the back. However, I personally do not believe in using weights on them. From a purely bodybuilding perspective, the goal is to make the shoulders look really wide with a nice taper to a narrow waist. Doing anything to make the waist thicker, even by building big and strong abs is a going to detract from the required goal of a narrow waist. They also can be treated like any other bodypar and trained only once a week, although I fall into the traditional trap of doing them more than once - I think because I don't use weights on them so I only exercise to keep them toned. If you are gaining size anyway, your natural increase in bodyweight will make you abs work harder. Bodybulding is about creating an illusion. If you add an inch to your shoulder width, it will also make your waist look smaller so don't forget the dumbell raises to the side to work the outer shoulder muscle

Localisation. I.e. will working one specific area result in fat loss in that area, e.g. by doing loads of abs will it burn more fat in that area. Personally I don't believe it does. If you do no other exercise and do 50 reps on abs then they will become toned, but you probably won't burn fat becase you have not got into a sweat. If you do 1000 reps on abs then you will probably be working alot harder and thus burn fat. However, I believe you will burn fat from all over your body not just you midrift.

In a nutshell, to loose the beer belly you need to reduce your fat levels overall. Its very true that people hold more fat in different areas of the body, I am the same. I hold far more fat in my torso than I do in my arms. But I know to loose the fat around the waist my whole bodyfat levels have to come down.

Ab exercises

Situps - the traditional ones where you hook you feet under something and pull yourself upright. These have been replaced with the crunches and these can be done in a manner of ways

CRUNCHES

a) put you feet up on the bed so you upper and lower legs are at 90degrees to each other and your thighs are perpendicular to the floor. Place your hands at the side of your head and roll your head and shoulders off the floor until your elbows touch your knees. You abs should be contracting tightly.

b)Don't rest your feet on anything. Just hold them in the position above in the air and crunch upwards.

c) as with conventional situps you can twist so your left elbow goes to the right knee and vice versa

d)keep your feet flat on the floor, your legs are at 90degrees still but your thighs are 45 degrees to the floor.

e) using a bench in the gym you can set the angle you lie. The steep the angle the harder it is.

LEG RAISES

a)Lying on your back (maybe holding something behind your head or placing you hands under your bum) raise your feet off the ground. Don't go past 60-70 degrees as this will take the tension off the abs.

b) hanging from a bar/garage rafters, raise you legs until they are parallel wit the floor. Depending on ow string your abs are bend your knees as this makes it easier.

c) When bending your knees on the way up you can twist side to side to train the muscles down the side of your abs.

TWISTS

I don't personally rate this but some people spend hours with a broom handle across their shoulders and twisting the torso from side to side, The hips and lower body stay stationary. This doesn't actually strengthen anything IMO and I believe people think it helps burn the fat locally(see above).

SIDE BENDS

Holding a Dumbell in each hand (or not) bend sideways as if you are trying to touch the outside of the knee. Pointless IMO.

USING A MACHINE

Kneely down facing away from a pulldown machine, reach above and grab the handle. Hold it behind your head and then crunch forward towards the floor. Vary the weight accordingly.

MY AB WORKOUT

If I have a good week I'll do this at the end of each workout. I do this non stop so I effectively do 50 reps. If I'm feeling particularly fruity I might repeat it

Crunches with feet in the air X 10
Crunches with feet in the air and a twist to the right X 10
Crunches with feet in the air and a twist to the left X 10
Crunches with feet on the floor X 10
Leg raises X 10