The general rule is, if you're not struggling with the last few reps, you're not forcing enough effort on your muscles for them to change. So yes, maintain proper, controlled form, but aim towards having to challenge yourself to finish those last reps. The tricky part comes from deciding how many sets and reps to complete for your exercise. Different rep/set combinations can be found as suggestions to achieve different goals, and even towards the same goals you'll find people suggesting different things. However, the most generally excepted default is a program which aims for sets performing 8-10 reps.
Also, you should not be exercising the same muscle groups day after day. Everytime you exercise a muscle, you are in fact breaking it down. It is only during rest that the muscle can rebuild itself, and if you trained correctly and eat correctly, grow stronger than before. You should be able to progressively lift more every time you do a weight exercise. If that's not the case, you over/underworked the muscle, and/or didn't give yourself enough time for recovery.
It is generally suggested to rest at least 48 hour before working the same muscle group again. The best way to do this is to exercise one muscle group on one weight training day, then another group on the next day, and so on usually performing a cycle. Most people split up weight training into three days a week working on three different muscle groups each day, which will give your muscle groups plenty of time to recover before the next exercise. If you want to spend less time doing muscle training, you could set it up for two days a week spending one day doing upper body and one day doing lower body.