By wildly do you mean that they're strafing left and right? If so then expect them to go right when they go left etc. You can get hints about where people want to go based on their playstyle. A lot of people favor directions and will tend to go to the right or left more of the time so that's always something important to look for. A lot of people will strafe very wide and do it frequently which is, for me at least, hard to expect when you're used to people doing the usual left, right, left, right. Many high-level players wait a while because they're more comfortable with waiting for a shot that they're pretty positive will hit rather than taking what can sometimes be a 50-50 chance.
Also take into consideration the environment. Although this plays a bigger factor ingame it's still important to take note of the possible places someone can land. Many players like just going high ground every time but others will change it up by purposefully landing off of it and disadvantaging their position in exchange for catching you off guard and maybe landing enough spam to knock you off too (think gully mid).
When it comes to juggling the main reason I miss double or triples is because I react too fast. The most important factor is to calm down and take a few milliseconds longer to line up the shot rather than flick and miss. And ya, aim as low as you can on the person so you can keep popping them up (if it would benefit you).