If you're having feedback from your lav mic, it means you are listening from speakers in a live sound setting while recording. The most effective way to control feedback from lav mics is by working on the speaker placement. This is because since most lav mics are omnidirectional, they are more prone to feedback since they pick sound from the speakers too. First, make sure that your speakers are on the room extremes. Actually, it's much easier to use lav mics with ceiling-mounted speakers. If those are not available, experiment with different speaker placements. The other option is to go for a cardioid lavalier mic. Although not common, they exist.