Maybe a lighting problem? what are you using? Other than that, bamboo cane and plastic tie, or similar sholud do.
If the lights are fluoro then you may not have them close enough to the top of the plants, should be about 3-6 inches from the top, otherwise you can cause the plants to "stretch" towards the light, if HPS then you should be ok? they will also strengthen if you use a fan, this will cause the stems to gently sway and ultimately become thicker and stronger. look at www.dopequest.com it has excellent details on growing techniques indoor and out. Have fun!