I play VP and BJ almost exclusively, but will occasionally play some slots just for fun and relaxation when I don't want to think, never to try to win much. Here's some random advice:
1. If you're a VP player, you probably won't want to be playing really serious money in a slot machine, the EV is always a lot lower than the crappiest VP. I'll put Franklins in a good VP machine all the time, but usually $5s or $10s in the slots.
2. Almost everyone agrees that in a multi-line machine it's much more important to cover all the lines than to play a few lines for multiple coins.
3. There's no way to determine how "loose" a slot machine is, unlike in VP. You can't just go by playing for a few spins, either, the variance is much too high and it would take maybe 100,000 pulls to really get a gut-level sense. Of course, you know about how slots are never "due" to hit, etc. There's probably also not much to the notion that the loosest slots are at the ends of banks, near high-visibility areas, etc.
4. You can get a notion of the volatility of a machine, by looking at the top jackpot. If it's huge, that's where most of the EV for the machine is, and it will be very volatile. If it's more modest, it will be less so, in general (think JoB vs. DB). On the video slots, maybe you can watch for a while and see if people get modest bonus rounds fairly frequently, or very high-yield bonus rounds fairly rarely, but again, it's hard to eyeball anything and know, since the variance is so high.
5. You're gonna get comped like crazy if you play a lot of slots. There's a notion that some casino computers will identify you as a "slots player" even if you have $5000/d coin-in playing good VP and $20/d playing slots; I haven't had that experience, and I think those days may be over.
6. Don't worry about "easy to understand and play" slots, they're all pretty easy and you can't make a mistake (by law, there can't be a skill element; not sure exactly how this law plays out, but I don't think you can screw up by not understanding what the choices are in the bonus round, for example).
7. You should probably just find a machine that you enjoy; that's the major thing for me. Maybe the newest, trendiest machines are tighter because of supply-and-demand, maybe they're not. But just find something that you like.