I've also recently purchased a fretless bass and I'll share some tips about staying in tune.
1. Practice slowly and with a tuner as a reference.
2. Use open strings when possible. This really helps with training your ear to hear the correct pitch.
3. Don't try anything fancy. You'll have plenty of time to play "Teen Town" later.
4. Realize that you won't be perfect and its part of what makes fretless so unique. You have the ability to let the notes "breathe" a bit. You can slide in or out of tune where it's appropriate.
5. Listen to some great fretless players. Jaco, Pino, Gary Willis, Tony Levin, Jack Bruce, Jimmy Haslip etc.
Hopefully these help. I'm getting more confident and find that playing fretless has had a really positive effect on my approach with my fretted bass as well. It kind of forces my left hand to not get lazy.