What is the best way to tighten abdominal muscles?

Sit ups are probably the best known and easiest to do and need no weights although there seem to be a few different ways of doing them.

One way I would suggest is to start with few repetitions and build up as your muscles get used to it.

Lay on your back and bend your knees( makes it easier on the back), perhaps with feet under something to keep feet on the floor.
Cross arms across your chest and raise your shoulders and upper half of your back of the floor. Don't sit up all the way as this will start to use other muscles and you want to concentrate on one group.
Try not to put hands behind your head as this will use your arm muscles as well. To start try a repetition of 10 at a time breathing in as you lift and out as you relax back down again.

Another technique is to stand up and suck your stomach muscles in then release them by 'snapping' them out again repeat this a few times

I know that people say sit-ups dont help but, i know they work for me and they probably work for you, too. I have a routine where i do atleast 20 sit-ups a day and in less than a month i found amazing results. All you have to do is atleast 20 sit-ups a day or for faster results or results in 1 week try 50-100 sit-ups a day

Actually sit ups are only one way to strengthen ab muscles and if you only focus on sit ups it won't do very much for you in the long run. You want to do core work and focus on the whole area to strengthen all your abdominal muslces as well as your back (which is very important for ab work).

http://www.cnn.com/HEALTH/library/SM/00071.html more info on the subject

Pilates and using an excercise ball are really beneficial for core work. Ask a personal trainer for specific excercises.

Sit-ups are probably the best BUT NEVER hold your feet down.
If you do, you run the risk of tearing or damaging your back muscles.
In the "old" days, holding the feet was the way sit-ups were done but we know better now.

Situps may be really good for the abs, but they are bad on you back later in life. Crunches help your abs tremendously, and save your back, by only lifting your shoulders off the ground. I do crunches, leg raises, and russian twists. They develop my core whilst keeping my back from hurting.

These tips really made a difference in the results of my workout. First,BREATHE!!! It isn't going to do much good if your muscles don't have the oxygen they need. And form is always essential-it's better to do fewer reps and do them properly than a bunch done sloppy. One trick is to imagine there's a string tied to the tip of your chin that pulls you upward toward the ceiling. Be sure to immobilze the lowwer half of your body to isolate the right group of muscles. But,doing something,moving,is the thing so Good job getting started!!

When people say tighten abdominal muscles, they really mean lose the fat covering the abs. I wanted to clarify this for one important reason.

Spot reduction will not burn fat from the stomach any differently then any other exercise that burns the same amount of calories. All fat must go through the same organ to be burned so it does not matter where the fat comes from..

Now I do crunches with a 25 pound plate on each shoulder, weighted leg lifts, twist weighted crunches, and never lost an inch on my waist, and I didn’t expect to.

I actually worry less about abdominal exercise when I am cutting fat. The primary focus of exercise is to keep the body from burning muscle instead of fat, to increase the amount of calories burned in a day, and do certain exercises (High Intensity Interval Training) that will increase your metabolism for the rest of the day.

In my opinion, the best way to tighten your abdominal muscles is to do the bicycle exercise. All you do is lay on your back and lay your legs straight but keep your legs above the ground a couple of inches. Then start paddling your feet in the air.
For more ab workouts go to - http://diet.ivillage.com/tone/tabs/0,,bw5w-2,00.html - and continue with the pages for MANY MORE.

I got flat abs by changing my diet and doing exercises that not only strengthen the abs, but the core as well.

I first did a liquid fast for a month which cleaned out my system (under the supervision of a nutritionist). In addition to the weight loss, my stomach started getting flat for the first time in my life. I always did abdominal exercises (stomach crunches, oblique crunches and exercises to strengthen the core), but they never had any effect until I fasted and changed my diet permanently! Now, the exercises seem to tone my stomach and I do them 3-4 times a week.

I cheat once in a while, but know I can gain it all back in less than half the time it took me to lose it. After a while it got easier. I don't crave the chewy fattening breads and cakes that often anymore.

There just wasn't any short cut for me. Good luck.