First off, understand that H-L-M training is naturally a great way to gain muscle mass. Full-body workouts (and I think that, overall, H-L-M training is the best full-body workout you can do) act as a muscle-inducing "trigger" (for lack of a better word). There's just something about training all of your muscle groups in the same session that makes you gain muscle faster than split routines. (Before some of you start crying foul at this point, realize that I'm not entirely opposed to split routines, as evidenced by my previous post on one-exercise-per-bodypart training. But you should only do certain styles of split training when you are ready. In other words, when you've laid a firm foundation of strength and mass via full-body workouts.)
The second reason that H-L-M training is so good for packing on the mass is because it allows you to train each muscle group frequently. The more frequently that you can train your muscle—and still recover between each session—the more muscle you will gain.
With that out of the way, let's get right to the point: several tips that help you get the most out of H-L-M training when trying to pack on the muscle.
Tip #1: Use Rep Ranges Conducive to Muscle Growth
If you read my previous post on "building the squat" then you know that H-L-M training works best when you vary your repetition ranges on a regular basis. And in the squat post, I recommended varying between sets of 5, sets of 3, sets of 8, and singles in different workouts. When hypertrophy is your goal it's best to take a slightly different approach, avoiding the singles.
Here is the rep scheme I recommend when gaining muscle:
weeks 1-3: 5 sets of 5 reps (this includes warm-ups)
weeks 4-6: 5 sets of 5/4/3/2/1 (plus warm-up sets)
weeks 7-9: 4 sets of 8 reps (this includes warm-ups)
weeks 10-12: 6 sets of 7/5/3/7/5/3 (plus warm-up sets)
I like this scheme because it contains more overall volume than a purely strength-oriented routine. When gaining muscle, remember that volume is good. Of course, you need to recover from the volume if you're going to gain any muscle, so that's the second thing I like about this scheme: the overall volume wavers throughout the 12 weeks. The first 3 weeks you're only performing 5 sets of 5—a scheme that's easy to recover from. The second 3 weeks sees a nice increase in volume—before you get to your first "work" set you should do 3 to 4 warm-up sets. The third 3 weeks sees volume go down slightly since you're only performing 4 total sets. And the fourth 3 weeks has the most volume— 6 "work" sets plus the required 3 to 4 warm-up sets.
Tip #2: Add Back-Off Sets When You're Ready
When your body is ready for it—in other words, when you can handle the extra workload—you should start adding back-off sets to all of your core exercises. When adding back-off sets, I recommend starting with 2 sets of 8 reps (unless it's on a day when you are performing 8s as your core repetition range). As you grow accustomed to the 2 sets of 8, just add an extra set. Once you reach the point that you're capable of handling 4 sets of 8 reps as back-off sets, you will need to start varying your repetitions. Start alternating between 4 sets of 8, 3 sets of 12, and 2 sets of 20 reps.
Back-off sets work because they allow you to easily increase your total workload—something else you need to do if you are to continue gaining muscle with H-L-M training.
Tip #3: Use Assistance Exercises Conducive to Muscle Growth
When training for strength, it's important to pick assistance exercise that aid in increasing your lifts. For instance, if your goal is a massive bench press, then your assistance exercises better include plenty of direct triceps work, lat work, and front delt work (in that order).
Gaining muscle mass is different. You want to select assistance exercises that work the most total muscle groups. You want the exercise that give the most "bang for your buck". If you want, let's say, a great-looking chest and massive arms (I'm using these two muscle groups as an example; they seem to be the ones most guys focus on) then don't waste your time with cable curls and flye movements (or, hell, anything cable for that matter). Instead stick with stuff such as dips, barbell curls, and close-grip chins.
Keep in mind, too, that you're not going to gain an appreciable amount of muscle mass without focusing on the muscle groups of your legs and your back. As much as you might love doing push-ups and barbell curls—and as good as those two exercises are as assistance movements—you need to realize that exercises such as one-legged squats, lunges, stiff-legged deadlifts, bent-over rows, and chins are even better.
Laying Down the Muscle Mass-Smack with a Sample Program
Now that we have the three "tips" out of the way that I think are the most important, let's get right down to business with a sample program. Keep in mind that this is only an example, you should vary things based on your needs and your level of strength-fitness. Also, in this workout I'm using a 5 sets of 5 program as an example, but make sure you vary reps just as our first tip recommends.
Squats: 5 sets of 5 reps, 2 sets of 8 reps
Bench Presses: 5 sets of 5 reps, 2 sets of 8 reps
Power Cleans: 5 sets of 5 reps, 2 sets of 8 reps
Bulgarian Squats: 3 sets of 15 reps (each leg)
Wide-Grip Chins: 4 sets of 8 reps
Dips: 4 sets of 8 reps
Squats: 5 sets of 5 reps
Bench Presses: 5 sets of 5 reps
Good Mornings: 4 sets of 8 reps
Close-Grip Chins: 4 sets of 8 reps
Squats: 4 sets of 5 reps, 1 set of 3 reps (set of 3 should be heavier than last set of 5 from Monday; if you don't understand, make sure you read my Dragon Door article on H-L-M training), 2 sets of 8 reps
Bench Presses: 4 sets of 5 reps, 1 set of 3 reps, 2 sets of 8 reps
Power Cleans: 4 sets of 5 reps, 1 set of 3 reps
Stiff-Legged Deadlifts: 4 sets of 8 reps
Dumbbell Bench Presses: 4 sets of 8 reps
Barbell Curls: 4 sets of 8 reps
Even though I haven't talked about it here, keep in mind that you need plenty of protein and calories to make any mass-gaining program work. My advice in this regard is to go to T-Muscle (see link to the right) and read several of their many good articles regarding nutrition and muscle mass.
Train heavy, train hard, and good luck.