It's not a stretch to say, you need to stretch

Whether you're an advanced, conditioned athlete, or a deconditioned, sedentary newbie with limited flexibility, it is important to STRETCH!!   Make sure the body is warm - a 5 minute jog, a 3 minute visit to the row machine, 15 plyometric jumps onto a box, or a few minute jump rope for heart routine should suffice- before u begin stretching out those all important muscles.  Being "warm" entails making sure the body temperature has risen, metabolism has sped up (isn't that great?),  fluid has hopefully lubed the often injured knees, and the sensitive rotator cuff has been disallowed to give you the "cold shoulder."  Fitness professionals everywhere use taffy to illustrate this point.  A piece of taffy, when cold, is more susceptible to break.  Although when warm, the taffy is more pliable and can bend with no problem.  Think of your body in this fashion.

Many famous, well respected fitness icons also endorse stretching between sets.  For example, between each set of chin-ups(or lat pulldowns), perform a hanging stretch on the chinning bar with a relaxed body - this helps equalize pressure between vertebral discs - or grab a vertical pillar such as the bars of steel that erect  the squat rack, clasp your hands together at shoulder level around the bar (your body will be in the shape of the "less than" symbol or alligator mouth we learned about in math class) and lean back... you should feel the stretch pretty intensely.  This should lead to more muscle growth as well... i.e. a wider back!

The last main point to mention here is the necessary evil we all must do, especially if weekly massages get too expensive... the myofascial release we know better as the 'ole foam roller!  Absolutely imperative, the foam roller is considered stretching, and an important stretch to be sure.  This can be performed practically anywhere at anytime, including before and after workouts - I would avoid rolling on the IT band (side of your thigh) unless you have a high tolerance for unnecessary pain!  
 
Greater flexibility, improved balance, and more functional muscles that are less prone to injury should follow!!!!!