Speaking in general, e-liquids which are dark colored or sweet (sugars) will gunk up coils faster than clear-colored or non-sweet eliquids.
If you change your own wicks, you can "dry burn" or pulse fire bare coils when you remove the old wick. Fire your coils for 3 - 4 seconds, enough for the coils to glow red, then stop. Repeat as necessary. They gunk will burn off the wire. A char of ash will remain, which you can rinse off under the faucet.