Many of the 270+ flavors I have (more than half) are in small (sample) 8mL glass vials and have too narrow an opening for glass eye droppers; so they necessitate the use of pipettes (disposable in my case: though I have used the long glass ones with a mechanical pump in the past). I am too cheap, and don't like to toss any more plastic into the landfill than I have to, so I try to wash and re-use them as much as possible.
For me; I have found that immediate rinsing in hot water right after use works best to extend their longevity. To make this as easy as possible, I have a (roughly) 1 quart thermos that I fill with very hot water (180-190F (?)) before each mixing session, set the cap on top without threading it (for convenience but still retain heat), and set it next to where I sit to mix. Keeping it loosely capped retains the heat quite nicely even if I am mixing for an hour or more. The larger volume of the thermos (compared to say a coffee cup) offers plenty of dilution to whatever flavors may be introduced into the water so I am not having to change the rinse water mid session.
Then, as I use a pipette, I simply lift the cap, give it 3 or 4 good squeezes in the hot water, give it a couple of extra squeezes out of the water to get as much water out as possible, shake it like an old-school mercury thermometer a couple of times with one or two more squeezes, and drop it back into the ready-to-use cup with the others. All good.
On the rare occasion I have a stubborn flavor concentrate (TFA's Banana-Ripe comes to mind), and I still have a residual odor, I will do a second rinse in pure-grain alcohol (AKA: ethyl alcohol). You can use any high proof neutral spirit like vodka or light/white rum too... the higher the proof the better... and the cheaper the better; name brands do nothing for you here. I have yet to have a concentrate able to cling after that combination

. I have some "disposables" that are at least a year old now with no odor and no residue. I generally don't toss them until the bulb cracks/splits nowadays.
Safety Note: I only use potable (drinkable) alcohol for this.
Never isopropyl (rubbing) alcohol, denatured alcohol, or any other alcohol that
is not meant for human consumption.
Another option I have read of people doing is to dedicate a pipette to each flavor and then simply rubber-band the pipette to the outside of the bottle when not in use for easy association. Then they don't have to be concerned with cross flavor transference.