Well you need to know that there are two type of stretching: Dynamic and Static Stretching

Dynamic stretching : is a form of stretching beneficial in sports utilizing momentum from form, and the momentum from static-active stretching strength.

Static stretching means a stretch is held in a challenging but comfortable position for a period of time, usually somewhere between 10 to 30 seconds.

This is a common problem when discussing stretching - nobody makes clear if they are talking about static or dynamic stretching.

Prior to your workout you should stick with only dynamic stretching (not static) and save the static stretching for after your workout. See, The effect of warm-up, static stretching and dynamic stretching on hamstring flexibility in previously injured subjects, Kieran O'Sullivan, et. al, BMC Musculoskeletal Disorders 2009, 10:37doi:10.1186/1471-2474-10-37 Study available online for free at: http://www.biomedcentral.com/1471-2474/10/37

This study examined the short-term effects of warm-up, static stretching and dynamic stretching on hamstring flexibility. The study found that:

1. an active aerobic warm-up significantly increased hamstring flexibility.
2. After warming-up (in this case a jogging for 5 minutes at a pace where a little breathlessness was experienced), static stretching further increased flexibility while dynamic stretching decreased flexibility.

The study found that increases in flexibility reduced after 15 minutes rest, but flexibility remained significantly greater than at baseline.

Long story short, active stretch before your workout and static after