Fishman M-200

This provides extremely clear and responsive sound, pretty much as good as having a decent mike pointed at the instrument but obviously still allowing you freedom of movement and with much less spill and risk of feedback.
It isn't a small investment though! Especially as it replaces the whole bridge, so fitting it involves basically setting up the mandolin from scratch again - unless you're very confident being your own technician, this is probably a professional job. I also found that my particular violin tapers too much towards the edges of the body for the Carpenter-style jack clamp, so the bloke fitting it had to replace it with a surface-mount socket and fit that.
All of that said, if you have a pure-acoustic mandolin that you really like the sound of, you want to be able to gig plugged in (particularly, though this pickup is good enough to record from too) and you don't mind making an investment, this is a tool that will serve you well.