Switching up your routine should be an absolutely essential part of any weight-training regimin. Many people work out by doing the exact same routines on the same days, lifting the same weights, with the same amount of time off in between, and never really notice much of a difference. The thing is, when you lift the exact same way for a long period of time, your body "gets used to it." What you need to do is "shock" your muscles to restart or increase their growth.
For example, if you're working your shoulders, and you've been doing military presses for as long as you can remember, switch it up next time and do some cable presses, or some Arnold presses with dumbbells. If you're running out of ideas for exercises or don't know what other methods you can go about shocking your muscles, take a look at the list of muscle groups on the right side of
this page and on the right side under "Exercises" click the appropriate muscle group for a list of different exercises, and links to an animation and description of the routine.
I like to do the same routine for about 6-8 weeks and then switch to something different. Some people switch more often, but this is what seems to work fine for me.
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As far as diet and your eating habits, I've said it several times, and I'll just continue to echo myself here.. If you wanna gain weight, if you wanna gain muscle, if you wanna get big - you've gotta eat big and you've gotta lift big. There's no other way about it. It takes discipline and dedication, but when you're ready to do it, you will find a way to make it all work. Food is your body's fuel, and protein is the building block for muscle. Without it, it won't grow. It's kind of like trying to start a fire without wood..
What you're going to absolutely have to do if you're serious about this is start getting your ass out of bed 20 minutes earlier (I know, I'm not a morning person AT ALL, believe me

) and preparing yourself something to eat. A bagel and a protein shake is good. Eggs, ham, turkey... Some form of protein is essential in the morning. Even if you're not hungry, just force it down. When you're sleeping, your body is in a catabolic state and it's being starved of nutrients. Your main goal should be to a) give it fuel for energy (some form of carbohydrate), and b) give it fuel for muscle building (some form of protein).
If you’re trying to gain weight, meal replacements should NOT be a substitute for a meal.. My thoughts on meal replacements are that they are good for ADDING meals to your diet. For example, if you’ve already had breakfast, and lunch, and you know you’re gonna eat dinner as soon as you get home, an MRP will be a good addition between meals to increase your calorie and protein intake and to keep your body going. As p-mo already stated, you're ideally going to want to feed yourself some form of protein every 4 hours or so. Beef jerkey (be careful with the sodium), tuna, an occasional protein shake.. I understand that being at work and school all day is extremely time consuming - and stressful, but that's only more reason for you to feed your body more often. When I was in school I used to take a cooler filled with food and leave it in a locker to snack on during the day. It was far from convenient to run to the locker between classes, but any way that helps you meet your goals and simplifies things for you helps.
And speaking of goals, you need to set goals for yourself. Aim to eat breakfast every day. You said you eat a big lunch and dinner.. Aim to squeeze one more meal in there somewhere. The more often you eat, the less your body will “starve,” and the better it will react to the training you are doing.
Nutrition should be the basic first step to any training program. If you're not eating right, you're not gonna get the gains you desire. PERIOD. Work on that and you will surely begin to see results. It's not gonna happen over night, but it's the beginning to the road ahead.
If you're going to begin taking supplements, I'd say supplementing your diet is the key for you. Protein is the way to go. MRP’s are ok, but like you said, they’re expensive. I’d go with a 100% whey protein, like Optimum Nutrition, and be certain to down a shake first thing in the morning, and post workout with sugar.
And just to reiterate, EATING MORE IS ESSENTIAL FOR GAINING WEIGHT.
