It's a good idea to stretch your muscles during a workout, between sets. The stretch performed should be specific to the muscle being worked. For example, if working your triceps muscles, then after each set, you should perform a
triceps stretch. By stretching them during the weight
lifting session, you will see increased flexibility and range of motion on subsequent days.
Never stretch before muscles and
connective tissue are warm and pliant. There is no need to stretch those muscles on days when they are not being worked—that is, unless you are doing aerobics or participating in some sporting activity (where many of your muscles are used). If this is the case, then it certainly is a good idea to perform a series of stretches to ensure reasonable range of motion during the activity. Still, they should be warm and flexible before stretching.