If you want good separation between your quadriceps and hamstrings, first make sure you are concentrating and that you’re really feeling weight on each rep. That is how you will create more detail. If you lift with explosive contractions, you won’t really fell the muscle, just the weight. You will get bigger but not necessarily better separation. For legs, use high repetitions on drop sets to fully engage and fatigue the muscles. Aim to do more than 15 reps per set so that you feel the burn. Plus, high rep training for legs helps to burn through more glycogen (stored sugars in the muscles) and total calories to help you get leaner overall.
This all depends on how long you have been training for. Basically, advanced techniques like forced reps or drop sets can be used about 3 months after you have started training because that is when you typically hit a plateau. So if you feel like you need something changed, if strength or muscle gains have come to a halt, then advanced techniques may be good for you to try. Forced reps, where you have a training partner help you through a few additional reps after hitting initial failure, are a good place to start as long as you do not overdo them.
Heavy incline barbell presses are a well-known chest-builder, but they can be dangerous if you are training alone. Done in the Smith machine, they will hit your upper pecs through a full range of motion with less risk of injury. You can work in some partials to train weaker parts of the movement. Finish off with a set of incline presses with a huge drop set, removing plates and getting right back under the bar each time you hit failure. This technique flushes the muscle with blood and takes you well past failure. Partials and drops on the incline Smith should help your chest a great deal.