Most plants stretch because they are not getting enough light. Move the plant closer to the light source, increase the light output of the light source or increase the number of light sources to help minimize plant stretch.
Light color can also affect the stem length. Plants want more of a blue color to grow. HPS lamps are rated at or near the 2000 Kelvin range; they are too red in color to stop the plant from stretching. Higher Kelvin ratings mean more blue in the output of the lamp. Indoor growers with plants in vegetative state should not be using solely High Pressure Sodium lighting. Metal Halide lamps come in a variety of Kelvin ratings.
It is my opinion that the perfect bloom Kelvin light is 3000K. If you are using HPS lighting exclusively, add some standard blue T-5 fluorescent lamps (5000-6500 Kelvin) to the growing area in both vegetative and bloom areas and log the difference in growth.
T-5 fluorescent lamps can now be purchased with a fixture that holds only one lamp. These allow you to add extra light to manipulate the total Kelvin illumination of the grow room. Adding two to four T-5 6500 Kelvin lamps per 600 or 1000 watt HPS lamp seems to do a pretty good job pushing you close to 3,000 Total Kelvin illuminations in Bloom.
Use a metal halide lamp or multiple T-5 lamps in the vegetative phase of your grow. When it is time to switch your plants to bloom, add a few single T-5 fluorescent lamps along with your HPS lamp and then turn the T-5 fluorescent lamps OFF 2 weeks before harvest. This method makes for a nice light transition to simulate spring to fall. Log your notes for later comparing between grows.
High amounts of Ammonia or Urea Nitrogen in your fertilizer can signal the plant to make cells without waiting for essential elements to be part of the cell make-up. This can result in plant elongation with weak stems and stalks. Combine this kind of fertilizer application with low light and/or exclusive HPS lamp use and you could have a recipe for disaster.
Even if your plant has already stretched out, you can still help it out by pruning the plant. For every pinch, 2 or more stems grow in its place.