Dynamic exercises are generally better than static stretching when it comes to warming up. Not only do they loosen things up without harming performance, but the right moves can even activate muscles and enhance performance. Here's one to try out.
This is very useful for the external and internal rotators of the hip, which tend to have some residual kinks no matter how well you warm-up. This will also loosen up the knees by acquainting them with the extended ranges that may or may not be encountered in the upcoming lifting session.
Bonus: They'll also give the lats and trunk a good stretch.