Heavy weights but not done perfectly?

If it's your last set for that exercise, and the last 2 reps, then it's fine to struggle since you're maxing out your muscles and pushing them to the limit. If you are struggling on the first set in the last 2 reps, then lower your weights. Your first 3 sets should be proper form with the weights. 4 sets of 8-12, so 1st set is some weight, the 2nd set is 5 pounds higher, or w/e, the 3rd set is higher than the 2nd, and the last set is the max you can handle. The first set should make you feel worked, the second a bit more, and the last you are pushing yourself. Rest 1 min or 90 seconds between sets. Good Luck.