Two solutions: a dripping atomizer or a Nano clearomizer.
Atomizers are juice delivery devices not too different from a cartomizer, but have no polyfill in them to hold
e-liquid. With an atomizer, one drops only few drops of juice into the atomizer and then vapes it. After a several puffs, a few more additional drops of juice will need to be added to continue
vaping. This ritual is repeated as long as the user vapes.
Many vapors prefer to use this method over others, as there is no wicking material involved which may affect the flavor of the juice. Many people also use this method to test out new sample flavors.
A video on how to use an atomizer:
ProVape Electronic Cigarette Atomizers & Cartomizers Refill and Instructional Video - YouTube
A Nano clearomizer is a smaller clearomizer in the size and shape of a cartomizer. As someone who doesn't drip on an atomizer, I find these to be useful for testing out new flavor samples. They only hold 1ml of juice, and are easily re-useable by simply removing the first flavor and adding the next. Easy screw off top like the regular clearomizers to fill. They use their own stock proprietary mouthpiece. Allow to sit for a few minutes after filling to allow the bottom-fed wick and coil to become wet before using.