Long term use of any type of laxative is not recommended. Natural or synthetic, the strong laxatives like senna that work by causing irritation in the bowels to induce a bowel movement, will cause dependence if used over extended periods of time.
I know it sounds basic, but please make sure you are consuming adequate amounts of water. I recommend people consume about ½ their body weight in fluid ounces up to 80oz per day (i.e.- if you weigh 150 lbs, try to consume 75 ounces of water daily). Also, make sure you are eating enough fiber.
Most people have heard this advice before and do not have mush success, however it is the most basic first step in keeping your bowels healthy.
If you are still experiencing issues after implementing these basic steps there are a few things to consider. I often find that a very common cause of chronic constipation is related to a low thyroid function. Make sure to have your doctor run basic blood tests to assess your thyroid. According to new standards, a TSH value over 3.0 could mean you have low thyroid function.
From the natural pharmacy, you can consider the use of Magnesium citrate as a gentler, less habit forming bowel stimulant. It acts by attracting water into the bowels which adds volume and can reflexively stimulate the natural muscular contractions needed for a bowel movement. In addition, magnesium is a very important nutrient needed to carry out thousands of reactions in the body. Vitamin C in high doses (3,000-10,000mg) will have the same stimulant effects on the bowel.