That's funny, I avoid playing open strings as I love the mwaah sound too much

, I want it in all notes. I use other instruments, or practice over records, to ensure I'm in tune.
It should be obvious, but I mention it anyway: Make sure the bass is correctly tuned and intonated. Even though it's possible to play in tune without that being ok, it will be a disadvantage for your muscle memory and it will take much longer time for you to learn to play in tune.
+1 to practicing scales, intervals etc... I'm still a newbie on fretless, but I've found practicing octaves all over the neck is very valuable and it's very easy to hear if they're in tune.