Side plank is one of those poses that is actually a lot harder than it looks. Planks don't look hard, but we dread having so many of them in our workouts, and side planks take away 1/2 of our stability in plank, so your darn right they are hard. Don't worry though there are plenty of ways to modify your side plank to make them more accessible to you, as well as ways to help improve yours.
The first step to helping your side plank is identifying what you need. Is it stability, strength, flexibility?
-Giving yourself a bigger base can help with stability. By dropping your bottom knee, you are giving yourself your knee, shin and two feet to help stabilize yourself rather than just one foot.
-You can also simply stagger the feet so that both feet are on the mat, instead of dropping to a knee.
-Another way to help give you stability in any pose is to shift your eyes. In a side plank the easiest way to find your balance is to turn your head towards the floor and ground yourself, by looking down at it.
-If you want to challenge your stability, you can lift your top leg. This can be done with one knee down.
-This can also be done from one foot down.
-You can also work with your eyes again to challenge your balance. Try turning your head up towards the ceiling and focusing on the tips of your fingers or staring at the ceiling. For an even greater challenge you can try closing the eyes (just be prepared for if you fall, some pillows or blankets would be nice to have in the fall zone.)
-Maybe flexibility is what you are struggling with. Wrist flexibility is a major problem with yogis, for a few reasons: Wrist stretching is not part of most classes, and lets face it, most of us don't have a home practice. A lot of us come to yoga from other sports, or from a work environment, where previous injuries may have occurred. As our bodies get older a lot of us develop complications is our wrists such as arthritis, which can make doing yoga uncomfortable. Any time your wrists present an issue you can take the pose down to your forearm. This will take pressure off the wrist joint as well as keep you from over stretching the wrist.
-One way to modify your side plank to increase your flexibility as well as your balance is to do a hand to big toe variation. Send the top leg into the air until the hand that's reaching up can easily reach it and hold.
-A great way to increase your leg strength is to make sure your legs are active in a pose. I like to place a block between my legs and squeeze.
By using any of these modifiers you will increase your ability to do this pose which will allow you to get stronger.