Well if you know your camelot system and what works with what you can predict your next mix, or where you want to be in a few mixes time.
Here's an example. Day & Night akesson Remix is an 11A (i think) and has 2 drops. There's one after the breakdown (electro bass) and then another with about 2mins to go i think. Anyway, i tried starting Miracle Club Mix (10A and first drop @ 2min30sec i think) at the first drop and the bass change at the miracle drop.
Structure wise its fine, and its in key. But because miracle is a higher key, you cant hear the vocals for dick (even with a bit of bass). So now ya know if you want to layer vocals, using a lower key is better because its louder and more powerful.
But thats all in key. Onto knowing HOW to mix in key. I find +1/-1, +2, +4/-4 and depending how you do it +6/-6 all work well.
+1/-1 is mixing in key.
+2 is mixing in a tune thats 2keys higher so you get a sort of energy lift with a decent bass change.
+4 is similar to plus +2, great for a definite change in the direction of your set as well as uplifting. -4 is good if yoou want a BIG bass drop, because its 4keys lower, and therefor louder (greeat for electro basslines).
+6/-6 is a bit more difficult - same principle as +4/-4 except you need to eq the highs more otherwise its a big change that can sound kinda flat (does that even make sense? haha). Although not the best, +6/-6 is good if you want to play some songs in key that are on otherside of key wheel.
The other ones; -2, +3/-3, +5/-5; i find clash too much and dont work as well.
However, i love my big bass changes - and using the key system in this way lends itself to that. If you wanted to layer basslines it can get very muddy very quickly. I also find i can progress a set much better, using transitions to help control energy level of my set.
Dissonance (out of key) helps build tension. As such you could do some out of key mixing to help build energy, create different basslines and sounds (by layering out of keys). FOr example mixing +2, then +4 then -1 then +1 purely from a technical point of view would mean you have a little energy lift, a bigger energy lift, the biggest energy lift and then a big bass line to start all over again.
Of course song selection is the most important, but knowing HOW to mix in key can help with creativity as well as giving your set a bit of direction
Hope this helps???