When it comes to putting on muscle mass, what methods have you found most effective?

I have a lot of passion for this question. Anthony is right if you want to keep it simple. If you do not want to dedicate yourself to doing all of those, then you need to skip it. Things to consider:
1.Body type plays a huge role. This does not mean that over time you can slowly get some gains.
2. Mass is easier with more calories. You have to decide what you want to look like. A lineman for football will put on size, mass and strength and not so much concern on fat as long as they can be quick and strong for 10 to 20 yards. If you want to go with less fat, it becomes a big chore to eat since the foods will be lower in fat and cleaner.
3. Lift big at least once a week for each body part. Periodization is important to stop the plateaus. If you are looking at strength, consider twice a week per body part. If you want to get good at something, you have to do it more to a certain degree. You don’t want to get hurt. Some bodies do have a harder time with the heavier weights.
4. Listen to your body-no matter what any of us tell you, you need to know what your body is capable of doing. If it does not feel right, back off.
5. Supplements can really help. Ones that you may want to consider:
Creatine
Protein
I am not a big fan of pre-workouts but some people need the boost-be careful with pre workouts. Always be careful of the product with stimulants.
I like BCAAs which come in good protein shakes along with supplements like Arginine that help with blood flow that can be found in Protein shakes also.