Planks!

 

Me doing a plank with hands down (notice: elbows and wrists are under shoulders, neck aligned with spine, heels pushed back slightly)
*And yes---I LOVE my new, round mat!

 I LOVE planks!  One big reason: it's a fabulous
MULTI-TASKER (as shown in this 2nd pic).  Notice in this plank what muscles are working hard to keep you stabilized in this position? 

Plank on elbows (notice: elbows under shoulders,
 neck aligned with spine, heels pushed long/back)

I know I've said it before, but it's worth repeating: FORM 1st!

Form: 
*Have feet hip distance apart
*Elbows under shoulders
*Neck STRAIGHT with spine
*Glide shoulders down the back and drop them into "imaginary back pockets"
*Engage the abs by pulling them up (picture a string attached to belly button and pull it up toward ceiling).  Engaging abs also helps stabilize the hips...don't rock the body!
*Apply some pressure into heels (by lightly pushing them back & feel elongating through the back of your leg)--when you do this you'll really feel everything-front to back/CORE-engage even more!!
*Hold in CORRECT form for as long as you can--working up to a minute to 1 1/2 minutes.
(Remember: don't get frustrated if you can only hold it for 15 seconds--it is better to hold it in the CORRECT form as mentioned here vs. a longer time incorrectly!)
Every few weeks try increasing your time until you can hold it for that minute!  Trust me, you will get there but it just takes time!!

Try a few a day (and they don't need to be done back to back.  You can do one in the morning and one at night.) 

It is one of the best things you can do for your core, body, spine, posture, etc. so try to do them several times during the week.  After several months I know you'll feel the benefits as I had.  Again, it takes time and won't happen overnight! 

Be sure to check out different varieties on YouTube or Google "planks". Power2theCore! :)