Things I've found helpful in my transition to Fretless. Some of these were carried over from my brief stint playing upright in College.
1. Your left hand technique is very important. If your left hand is sloppy on fretted, your fretless learning is going to be more complicated. Try to get your finger perpendicular to the strings, your thumb placement is important for consistent finger positioning and the ability to use vibrato. This will also help you avoid your hand creeping up or down the neck when playing in a single position...it happens. You won't feel like you're moving, but sure enough, you'll look over and your hand will have slid one way or the other a bit and then you wonder why your ear didn't catch it.
2. This has been said before, but use open strings to check yourself. This is an old upright technique. Sometimes if I'm playing, for instance, a D on the A string for more than a passing note, I'll also lightly pluck the D string to make sure I'm in a good spot. If you're practicing and want to reference, there's also harmonics that can either tell you where you are pitch-wise, but also act as landmarks on the neck. So, if you want to make sure you're landing on the right spot, check if there's a harmonic under that note you can check yourself with.
3. When Shifting, I think to myself "put your first finger where your third finger is" or "put your pinky one half step below where your first finger is" and I find it helps my brain move my hand to the right position. I do reference my fingerboard quite often, but I really try to play without looking at the board too much, unless I'm soloing. I like to try to make eye contact with the audience, but I will sneak peeks to check after a shift.
4. For improving my ear, I found this app called Chordbot, it's on Android and Apple. It lets you set up a chord sequence, and you can have it play chords, or arpeggios over that sequence with a variety of different instruments, in different registers, and move what note is in the low end. You can also adjust the tempo. I usually set each chord to go 4 or more bars before changing to the next chord.
When doing this, I run my signal through my Boss TU-3 pedal (just go out the bypass) and you can leave it on all the time), then out to my amp.
I'll then start playing the roots of the chords in various positions on the neck, when I think I've got it dead on, I'll reference my tuner and see how far off I am, then adjust accordingly and really listen. Then I'll move onto the next chord.
Once I've done that for a while, I'll start playing arpeggios so I get used to hearing the different "important" notes of the chord in reference to the chord being played, so it's more in context with what you'd experience in a live settings.
then I'll move on to just playing the 3rd, or the 5th, or any of the other notes in the scale for that chord to see if I can get that to land in tune, all the while I'm referencing my tuner ONLY AFTER I think I'm on pitch.
I found this quickly helped my intonation improve drastically to where I'm rarely more than 2 or 3 cents out, and man does it feel good when you nail it.
5. In regard to some of the other comments, I use a hybrid 1,2,4 and Finger-per-fret, I base it on what the need is. I favor the 1,2, 4, but there are times when the line is much easier with all 4 fingers. Basically, on my fretless, I try to shift as little a possible. This is helped by the 5 and 6 string fretlesses I play, but my upright teacher taught me that every shift is an opportunity to be out of tune.
6. Remember that while you tune, and hopefully everyone you play with is tuning, unless you have a fixed pitch instrument like a keyboard, there will be variations, and it's more important to play in tune with your group, than to be technically correct. This is one case where that is NOT the best kind of correct, because even thought your tuner might show you dead on, you'll sound off because the majority rules. If you have a keyboard, then you want to be in tune with them, and let the rest of the band hang out to dry. Guitars go out of tune pretty quickly, and if they have certain types of tremolos, it's even worse, and because the majority of guitars are fretted, they're locked into what their tuning is until the end of the song. (unless you have a guitarist with a great ear and the ability to play one-handed while he tunes cross-body with the other).
These are just my experiences, your mileage may vary. Music is an individual journey, and what works for one person may not work for the next. The important part is that you get joy from it.
