I'm inclined to think that the reason you keep turning it up is because you're not getting enough vapor for your tastes. A given tank can only produce as much vapor as available airflow and juice volume will allow. Adding more voltage to the mix just causes things to burn.
Firstly, you don't always have to replace the coil. Sometimes (not always), removing the coil from the tank (no need to dump your juice) and running it under warm water (nothing else) will do the trick and surprise you with the amount of flavor and vapor it produces after. Remember to dab it with a dry paper towel afterwards to remove extra water. Some people use alcohol for this, but I've never found that to be necessary.
Next, I'm assuming here (because you didn't say) that your tank is a Protank III. Switching to something with adjustable airflow will make things better for you. I'd recommend the Kanger Aerotank (mini, regular, or mega, all good) or the Nautilus Aspire.
Lastly, and something that I believe gets overlooked all the time: Your tip. A wide-bore tip can actually make a difference. I can't stand the standard tip on the Aerotank Mega.. To me, it's like drinking a cola through a coffee stir straw.
