Once a program and strength workouts have been put into place the next question my clients often ask is, "How much weight should I lift?". While my initial answer is something like, "More than you think you can lift.", the truth is if you want to be successful in the weight room, knowing how much weight to use per a given exercise is a skill you'll have to develop for yourself. Sure, a personal trainer can make suggestions and encourage you to take off/add more weight but in the end you need to be responsible for being in tune with your body enough to know when you can kick it up a notch, when you should dial it down, and when it's just right.
I'm going to assume that if you're following a more traditional strength training program your workouts are organized into recommended sets and repetitions. There are formulations and calculations to help you figure weights based on percentages of your one rep maximum weight. However, I would highly recommend one rep maximum tests be executed with a knowledgeable trainer/coach or for experience lifters. Regardless of your program/goals here a few general guidelines to help YOU decide how much weight you should be lifting.
Form first: Hands down the most important factor in determining your weight selection should be form. You must be able to maintain proper throughout the entire set. If at any point your form breaks down and/or you adjust your range of motion during the set (i.e half way down on squats) to cheat the exercise you need to reduce your load. Performing movements with poor form or reduced range of motion can lead to injuries, lack of progress, and above all it's just a waste or your time.
Channel your inner Goldilocks: Remember Goldilocks? Too hot, too cold, just right? It might take a little trial and error but try to find that sweet spot between, "This is a cake walk" and "Oh my God, I'm gonna die.". If you finish a set and feel like you have multiple reps (two or more) left in you, add weight! Unless otherwise noted on your workout, aim to finish a set feeling like you have one to two reps left in the tank. For example, if your workout consists of 5 sets of 5 and after the first set you feel like you could have done 8 or more reps, consider adding weight. The key here is to be honest with yourself. Only you can determine your level of exertion and by stopping short you're only cheating yourself.
The follow up question to this usually pertains to how much weight one should add. As a general rule of thumb, larger, more compound movements (squat, deadlift, etc) you can add more generous amounts of weight (try 10% of your initial load) where as isolation exercises might only allow for smaller jumps in weight. When in doubt, add lighter amounts of weight and build up.
Put your ego aside: Ensuring that guideline number one is met sometimes means putting your ego aside and reducing weight to perform a movement with proper form. Do not feel obligated to finish a set when the weight is either too heavy or too light. As soon as you realize the weight is not appropriate, stop, adjust the weight as needed and start again. I give you permission to stop mid-set and re-load. Don't worry about people watching you, taking the time to be certain a weight is appropriate for you is important and nothing to be embarrassed about.
In addition don't try to keep up with other lifters by using the same weights. Just because your friend used 30 pound dumbbells for a overhead press doesn't mean you have to. After all, how will you ever figure out how strong you really are if you just play tag along with your training partner? Find a weight that's right for you.
Actually rest during rest allotted rest periods: If you're lifting history consists of flying through workouts with light dumbbells you might find working with weights that are challenging (but not impossible) a little more uncomfortable. You should find yourself working harder during the lifts and therefore need to use allotted rest times for…..REST! If your workout program does not specify how much rest between sets, typically two to three minutes should suffice but if you need more, take more as heavier lifting, generally, should not be rushed. If you consistently find that you can move from set to set with little to no rest or muscle fatigue, most likely you can move up in weight.
Sometimes you've got it, sometimes you don't: Just like in life, in the weight room sometimes you're a super hero and other times you are super wiped. Either way, don't let your weights from last week, last month or even yesterday determine what weight you choose for your next workout. Use your training log as a guideline but let feedback from your body ultimately decide your weight.