plank form

Wrists, more importantly make sure you maintain a neutral spine, if your hips are too far in or too far out its not going to hit your core properly. I definitely recommend doing it on gymnastic rings if you have them available

They're both effective. I've been taught that on your forearms is an "ab hold" and on your hands is a plank. But, again, either is going to be a good core exercise. Why not do them both?

Supposedly on your elbows is the "hard" version of a plank although I'm much better on my elbows than I am on my wrists.

Either way you need to remember to stuck your bellybutton into your spine, squeeze those butt cheeks together and remain straight as a board, not lifting your hips up too high or dropping them too low. Do it in front of a mirror or at the gym and ask someone if you're doing it correctly. I plank for 1 minute and then rest for 10 seconds in child's post and then plank again and rest again etc etc.

Once you get steady enough, you can do up/down plank...start on your elbows and lift yourself up onto your hands and then back down to your elbows and back up etc. Or do spiderman while planking, it works your obliques: plank in whichever form you feel comfortable the move one leg up and out to the side with a bent knee and then the other.

http://youtu.be/DVXT7xBA_jU

http://youtu.be/L4oFJRDAU4Q

Usually I encourage beginners to hold plank on their forearms then as they get stronger, their wrists. It's also a good exercise to move from forearm to wrist then back again within a few seconds of each other; challenging but really strengthens your core & shoulders. Hope that helps

this

I believe the yoga pose is on hands, but they both work your core. I actually find it easier to hold a plank on my hands, but I think most find forearms easier. Either way, just do it. There are lots of plank challenges available online. Try one! If you are worried you are using the wrong form, switch it up. Or do the up/down exercise that @innocence suggests--we do that every couple of weeks in my circuit training class and it's great. Switch hands to make sure you are exercising your entire core.

I have read in several places lately that doing planks is actually a better core exercise than crunches, and it's certainly easier on your neck (I have a tendency to try to pull my neck doing crunches).

you can plank on either your palms (arms straight) or your forarms as far as i know. just make sure you dont drop your hips or stick your bum in the air too much

Which one more effective?

They both are. Some people have to start on their forearms for more stability until their core is strong enough to move to straight arm. Try PLANK UPS!!! It's starting in the forearm plank, then without moving your hips/butt too much, push up to your hands/palms in the high plank position. Then eventually you will be able to do side planks. Here are some images to help.

http://www.bing.com/images/search?q=plank+ups&id=438DFA5047CD5A9BCDF4DC29E2E2E4D0CAA8CD26&FORM=IQFRBA#

you can plank on either your palms (arms straight) or your forarms as far as i know. just make sure you dont drop your hips or stick your bum in the air too much

Which one more effective?

elbows as you are lower to the ground. also try side planks or Tplanks. (on one elbow with your body facing a wall instead of the floor)
Happy planking

And this (plank ups)

I was just going to suggest this - I call them plank-to-push-up. Absolute killer for upper body and core, far, far harder than it looks.