Buy low - sell high
That's a good one. My other favorite is know when to hold them, know when to fold them.
Buying stuff that is flat and low volume is something I do a lot. That said, I have a list of the top 200 coins I will touch and those I will not touch. Also I look for things that have a bit of history, I don't like to mess around with something that is younger than 6 months if there are too many unknowns.
Yeah, I generally make sure something has stuck around for a little bit as well, as there are heaps of scams out there imo. I don't have XRP ever.
No XRP? That's been one of my most profitable coins. I bought it simply because alt coiners hate it so much, but it appeals to institutions and those not so familar. Yeah I know its separate from ripple, also it costs money to make a wallet...which I think is unique (hive does have a paid wallet option too and it's also awesome).
That said out of the big ones I shun monero, bch, bsv and dash.
I used to trade XRP back before I knew what any of them were. I have a little Monero, haven't seen or heard from Dash in forever and never got into the Bitcoin forks.