My useful tip - for any instrument - is: practice, practice and practice again. and then when you've practiced, practice some more.

One tip I got from my old mate Alan Day, when pootling around with an Anglo concertina was: concentrate on getting an equal distribution of sound/volume on the push AND the pull. He could always tell if someone was a beginner by the fluctuating volume on the pushing and pulling.

As far as fiddle fingering is concerned, my own teacher also advised me to keep fingers down where possible - not only for lessening unnecessary finger movement but also for retaining position. Guitars have frets and fiddles don't, so finger positions are crucial for intonation. Interestingly, in a Chris Newman guitar workshop, Chris advised taking fingers off rather than leaving them on. We then all had an interesting conversation about contrasting fiddle and guitar techniques!

My tip for tenor guitar players: you can get away with the first 3 fingers on a mandolin for much of the time, but use all four fingers is a must for the longer scale tenor (both tuned in 5ths). So, exercise that little finger because it's going to work harder. [See my YT video Playing the Tenor Guitar (CGDA).

Finally, a controversial tip for fingerpicking guitarists: DON'T rest your little finger on the face of the guitar - it restricts movement. And, YES, I know that people like Merle Travis, Woody Mann and Duck Baker rest their little fingers... :-)