Um. I don't really beta test so much as just occasionally play and leave my thoughts on various incomplete games I like the look of enough to play so take my words with a grain of salt but I don't think its all that different from advertising your game in general.
Just do all the 'usual' things like have a decent amount of screenshots showing off your tile work, what kind of visuals you're using and/or new mechanics you've added to the game, have a list of intended changes and features, and a blurb about the plot and/or tone of your game, and then after have at the bottom that you're looking for Beta testers and would really appreciate feedback on balance and story.
Oh! As for showing that you're still actively motivated and still making the game, just a folder of 'versions' and the date they were each posted as well as a 'change log' can be really great at getting across you're still working on the game and it isn't dead.
Updates/developers logs in general (even just little random stuff like '[date] Got this move working today =D' or 'yay. more coding. the numbers consume me') are hugely encouraging when it comes to knowing that both you and the game are still alive and that any beta testing actually has a point to it. Don't know how viable that sort of thing is on these forum type posts but idk, a 'I'm still working' journal folder that gets updated every few months seems doable~
Might help to also have a banner advertising your game (including a link) and your need for beta testers attached to your signature as well, since if nothing else that might make the odd person you interact with curious. (Says I who is now curious but apparently sucks at finding people's games by looking through their profile XDDD)
And yeah~ If you've done all that and are still really struggling to pull in beta testers then... I don't know. Might be a case of 'not enough have seen those chances yet' or 'might genuinely need to do something to make the game standout visually or mechanically more' but regardless I hope this ramble helps you a bit~ <3
