So lots of people have mentioned 'reseting' floors here - is that just a mean of exiting the mines and coming back in again, or is there another technique? Excuse my naivity, but I only hav my 10yr old to talk to about SDV and he knows less that me.
Yes, that's just exiting and going back. I think you need to wait for the clock to change before you go back down, otherwise you go back to the same floor.
Two things change the spawns on a given floor:
1. Waiting for the clock to change, as
@Ereo mentioned. This is referred to as a "tick".
2. Taking either the ladder or the elevator back to floor 0.
So the fastest way to reset floors is to spam the elevator buttons until they break! :D
Example:
1. Decide what you are looking for. If ores, what type? If monsters, what type? In this case, we are here for Green Slimes.
2. If ores, head to floors that end in 0 in the ore's respective floor set. Ores won't spawn on floors with elevators (5, 15, etc.), but floors ending in 0 always have a ladder at the entrance (excluding 120), so take the elevator to these floors. Keep in mind that Gem Nodes can still spawn on -0 floors. In this case, we don't want ores.
3. If monsters, head to floors that end in 5. Even though you won't get any ores, monsters will still spawn. In this case, floors 5 and 15 are the best (25 also spawns Grubs, reducing the number of Slimes).
4. Take the elevator to the first in the floor set (5), kill any monsters near the entrance. Don't take ladders, return to the lift. Repeat for the second (and third, if available) floors in the set.
5. If one tick has passed, repeat step 4. If not, take the elevator back up to floor 0, and then repeat step 4.
6. Continue until your goal has been achieved.
I think it's best left up to personal opinion how far you are prepared to leave the entrance for monsters. My rule of thumb is if it's on screen when you're at minimum zoom level (75%), then it is worth it. But like I said, this heavily depends on play style.
Hope this helps!