I habitual stab at the switch to make sure they are cancelled.
Other than that, I recall when Honda came out with the CB400 four it had a buzzer that beeped in unison with the turn signals. You can probably pick up a cheap, 12V piezoelectric buzzer from an electronics store and find a good place to stash it, like under the seat. The most critical situation is when your turn signal is on at an intersection (left on from a previous turn), but you don't intend to turn. You should always be able to hear the buzzer in that situation. A more expensive solution, as stated earlier, is the Kizan Signal Minder, but it's also a bit more involved because you have to modify the turn signal switch by adding internal springs.