You shouldnt have your jammer on all the time. It will keep that from happening.
Though if you only turn it on once he fires, it defeats some of what ECM does. One of the nice things it does, is mask your range.
Usually I wait for a spike on RWR or TEWS. And only if I am up high enough to be in danger.
Also, you should learn to read the TEWS like the RWR. Its harder since you dont have a signal strength, but you should get a hang of knowing when hes close enough to fire. If the spike appears to be close enough to fire, turn off your jammer, if he had you locked, it will break is lock, and he will have to re-lock you. Since the lock breaks, its a good idea to try and change altitude quickly. That way if his scan cone isnt right, he wont see you and you can get closer, to get a 120 off.