Should I lift weights before or after soccer practice?

Ideally, you should lift weight following regular practice on scheduled resistance training days.  Practice is your first priority during the soccer season and performance training should integrate into the practice schedule.  If your schedule only allows time for you to lift weights prior to practice, then use this time in the weightroom as a warm-up to prepare yourself for practice.  Be sure to avoid pre-practice fatigue by lessening the number sets or repetitions, use lighter weight, and consider performing alternate exercises.