For pure strength look at using olympic style lifts such as deadlifts, squats, clean and presses, hang cleans etc. Use heavy weights and low reps of around 1-3. This will help towards strength rather than Muscle hypertrophy. For powwer look at plyometric training doing things such as medicine ball slams/throws, box jumps etc. Spend 10-20 seconds performing the exercises using maximum force. Also if your trying to avoid increasing your weight then keep an eye on your diet and do cardio training alongside your resistance work.