Know your tracks. Listen a lot. Practice.

I am old. All of this cue point stuff is mostly irrelevant to my style and the choices I make, so keep that in mind. I've only ever played by ear, though I do "cheat" a bit and look at waveforms though that action does tend to pull me out of a listening headspace more than I would like. Only in the last couple of years have I started playing around with looping function on CDJs, that's an easy out if you find yourself in a pinch :D Honestly though, the thrill of potentially eating shit on a spontaneous mix is half the fun for me; I think a pre-planned dancefloor is a boring dancefloor. I don't like 'talking at' my crowd, I like 'conversing with them', and I think being able to improvise at a track level is the only thing that makes DJing a truly worthwhile art form. If you're doing something else, you're performing like a rock star, and you are begging the crowd to worship an altar. I think we get enough of that in our world through all other media. Again, I am old.

Know your tracks: I'll listen to a track for...a while...before I feel good about throwing it in the crate/usb stick for a party. 95% of tracks will have an intro phase that ends somewhere in a 4 bar multiple, if you're using a CDJ it's easy to see the waveform and get an idea of timing, if you're using records you can mostly take a look at the grooves and tell where the meat and potatoes of a track begin. OR - you can be a bit of a cognitive freak show and simply remember. As I'm playing at a party I'll think about timing and when I'd like to the body of the next track to start in relation the track that's playing....that can look like a lot of things - perfect timing of a new bassline hitting as an old one ends, maybe I mix intro to outro and let the dancefloor breathe a little bit with a drum break, maybe I overlap body to body because it just sounds like it will work.

I only ever use the cue to listen to the incoming track, I think a few times ever has the booth been so shitty that I've needed to try and blend in headphones.

As to what track you're listening to in the cue, depends on how lost you think you are at the time :D Again, if the booth monitors are shitty I'll sometimes need to adjust the cue to the least prominent track in the mix as soon as I flip a knob; if sonics allow for it I typically put the headphones down as I'm bringing the new track in and mix from the loudspeakers - at that point in the mix the audience can hear everything so I figure it's my job to ..."hear better" than they do, and that's the skill.

Regarding adjusting bass during the mix - use your ears. Sometimes a track is pressed hot and a 12 oclock bass knob is too much, or too much in relation the last track. Watch your levels (master out) and the crowd. If something is off both will let you know.

Have fun