Here is an easy way to do this. All you will need to do this, the ground. Enter the Strongfirst Plank.
Most people plank for time to get stronger which is just performing an endurance exercise. This does nothing for getting stronger. The plank detailed in the following develops the ability to achieve overall tightness. This learned skill can then be demonstrated in greater strength. According to Bret Contreras, the Strongfirst Plank fires the internal obliques two times, the abs three times, and the external obliques four times as hard compared to the traditional plank. More bang for your buck especially considering how simple it is. Here is how to perform it.
Let us first start by getting into the traditional plank position. Forearms on the ground, parallel with each other, and your hands making fists. Elbows need to be slightly in front of your shoulder joint. Your entire back and legs will be in a straight line from your heels to your head. Now that we are set up correctly, let's begin to tense our whole body from the neck down. Begin to push your heels away, flex your thighs by squeezing your kneecaps up, squeeze your glutes and "crunch" down your abs. To top it off, drive your elbows toward toward your feet mimicking a pullup.. Your armpits should feel tight and this tightness should go further into your midsection. If done properly, there should be no change in body position and soon you will realize that you cannot hold this position for long. That is the point. All that you need to do is hold it for 10 seconds. Remember don't try to endure, but tense as strongly as you can. That is what makes this exercise as effective as it is.
Here is the prescription for using the the Strongfirst Plank.
-3-5 sets of 10 second contractions
-3-5 minutes of rest in between sets or throughout the day
Do this consistently for a month and you will feel a difference.