If you ask 100 different runners about stretching, you'll likely get 100 different opinions.  Some will advocate, spending 30 or more minutes a day working on flexibility; others will say that stretching, at best, is a waste of time,  and, at worst, is counter productive.

On top of that, there's a number of different types of stretching: passive, ballastic, PNF, etc. 

Here are a few thoughts to help you make sense of the stretching conudrum:

1. You CAN overdo it.  Yes it's possible to be too flexible.  If you stretch well beyond the normal range of motion, you're suseptitable to excessive movement around the joint, which can lead to loss of power or over use injury.  Your stretching routine needs to be specific to running.
2. Don't stretch an injured muscle. Resist the temptation to "yard on" that sore hamstring.  You're more likely to slow the healing process.  Let the tissues heal.
3.Don't forget about the IT band.  Every high school gym teacher and coach will emphasize hamstring flexibility for runners, but few will stress IT band flexibility.  Tight IT bands are the source of many running knee injuries.  You're advised to find a good IT stretch and do it daily.
4. Warm up, stretch a little, then workout, then stretch a lot. Ideally, you should jog for about 10 minutes, and then stop and stretch for a bit.  After that, proceed to the main workout, cool down, and then do a more extensive stretching routine.  Stretching a "warm" muscle is much more productive than stretching a "cold" muscle.
5. Don't be afraid of a bit of dynamic flexiblity. Sprinter's drills and a few mild pylometric exercies are a great ways to improve your range of motion.
6. Get a PT or massage therapist to show you a few things. These folks can diagnose your muscles imbalances and prescribe specific stretches and exercises to counteract your weaknesses.

And speaking of PTs, Gerard Hartman, Paula Radcliffe's PT, has penned an interesting article about Active Isolated Stretching.  With the AIS approach, stretches are held for a few seconds and repeated for 8 to 12 reps. If you've been struggling with maintaining or improving your flexiblity, you may want to try AIS.