Very often you will find the first discard an opponent makes will be from his five card or longer suit. Unless declarer or dummy also have five of this suit, it is almost always a safe discard. Similarly, your opponents will almost never discard from a four card suit first unless they are sure that declarer can't have four himself. It would be dangerous and may enable declarer to set up the suit when he otherwise wouldn't have been able to. A pitch from a 3 card suit or a 5 card suit would almost always be preferable.
How can you take advantage of this at the table? Well, recently I had K943 of clubs in dummy, and AQT in my hand. When I ran a side suit, RHO was forced to pitch on the third round and chose to pitch a club (looking at the 4 clubs in dummy). I found this to be unusual as with three small he would worry about picking up his partner's queen if I had something like AJT, and he would also probably have a better pitch available. With 4 clubs he would be worried that the pitch might cost a trick. I had a strong opinion that RHO had just made a normal "lazy" pitch from his five card suit, so I cashed the ace and then led small to the ten. I was gratified when LHO showed out.
Another interesting pattern people have is pitching down to equal lengths in two suits whenver possible. For instance, in a 4 card ending with only small cards people are much more likely to come down to 2-2 than 3-1. I'm not sure why this is, but it happens very often. If you don't believe me, try watching what your opponents do or even what you do yourself. You might be surprised. It's not immediately clear how this knowledge will help you, but I once had a spectacular hand that I guessed correctly using this inference.
RHO opens 2 and you overcall 2N. LHO bids 3, and partner raises you to 3N which is pretty ludicrous but there's nothing you can do about it now. You have to make 3N.
LHO leads a spade to his partner's ace, and RHO plays a spade to LHO's king. A spade comes back and you win the queen, having pitched 3 hearts from dummy. You know your opponents and they are not sophisticated enough to have given any suit preference signals with their spades so there is no inference there. When you run the clubs RHO pitches a low heart, a low diamond, and a spade. LHO pitches 2 low hearts followed by a low diamond. We have 8 tricks and need to guess which suit to finesse in order to make our contract.
If LHO has both kings we are doomed, and if RHO has both then we are cold. So we assume the kings are split. RHO probably would not stiff his king, even though it's free that would be very deceptive and would violate another pattern people have; keeping guarded honors unless they have to unguard them. So we can assume RHO is 6232 or 6322 and kept Kx in his 3 card suit and a stiff in his other suit. If RHO was 6322 that would give LHO 4342. Pitching 2 hearts and then a diamond would be very much against the pattern I described earlier. Much more likely is that he came down to 2-2 in the red suits. That would give him 4432, and RHO 6232. That means RHO started with the K, so you take the diamond finesse. When that works we have 10 tricks.
Had LHO pitched 2 diamonds and a heart he would have gotten me. Remember randomness in these situations is critical, so defy your human impulses and don't discard with auto-pilot on. Especially when you know you will have to make more than one pitch, figure out what ALL of them will be and throw your shorter suit first every now and then.