The obvious questions is why are you tense? is it fear because of pain you have when you have bowl movement? do you have a fissure? Do you suffer from hard feces that cause pain?
If your feces are normal then the easy way to relax the sphincter is by pressing hard under the tail bone upwards, about 45 degrees into the body, for 30-60 seconds. This point is commonly used in TCM and I found it very helpful.
Another way is to adjust your posture while sitting - bring a bucket and place your feet on it. This will simulate squatting. Any object that will elevate your feet to about the same level as your butt, will make your bowl movements much easier, natural and nearly effortless.