I have 3 Beams on my Anaconda that are G5 Efficient (others are modded MCs) and it's rare that I overheat or get self inflicted thermal damage.
Thing is, stop firing your beams when the target is out of it's firing arc or range. If you have turreted or gimballed weapons, firing them on targets that are past 2Km make them ineffective and tend to miss because of jitter. So control your fire rate
Another factor that may indirectly affect your heat or thermal levels is your other modules, if they also generate lots of heat then it can add to your overall heat levels. Getting A-rated modules is very expensive but these A-rated modules are the most efficient on heat handling. E, D, C rated modules are bad with heat efficiency. B-rated modules are so-so but they are your alternative to the expensive A-rated ones (although B-rated are the heaviest).
Also, get Clean Drives instead of Dirty Drives or if your power requirement isn't very high, get a Low Emissions mod on your power plant
