If you have been doing weights for a while, use 'tempo training' to smash through your plateau. Tempo training means you change the pace at which you perform each rep. It helps 'shock' the muscles out of plateau and back into growth.
For example, with a bicep curl, instead of spending 1 second for the eccentric phase (arm straightening) followed by 1 second for the concentric phase (the curl), try different timings.
A popular 'tempo' I love is 2-2-4. With a bicep curl, that means 2 seconds for arm straightening, a 2 second pause at the bottom, and then 4 seconds for the curl. Your biceps will burn and you will definitely feel the difference.
You can do this for many exercises, and after several weeks, try changing the ‘tempo’ yet again.
Note: If you change to tempo training you may need to drop the weight so you can sustain 8-12 reps with good form.
Please remember that technique is the most important factor for staying safe and getting results. Questions? :-)