while the combo for most players is to Q, land the hook and then flay them closer to you
a much more consistent combo I've found is to walk up to them (usually the opposing adc), do a reverse flay (E), then immediately follow with death sentence (Q). the knockback puts them in point blank range and the slow component makes it a guaranteed hook, so you get maximum value out of that 1.5 second stun.
maxing out cdr, you can hook a guy for 1.5 seconds every 6 seconds. this is because death sentence refunds some of it's cooldown if you actually land it.
thresh has an attack range of 450, which is 100 units shorter than most ranged champions who have 550. I believe the flay ability also has around the same range as your attack range, so if you find yourself capable of auto harassing, maybe try to do the E->Q combo.
your Dark Passage (W) lantern provides vision where you cast it. If the enemy is fed enough to kill you when you are alone warding, you can throw the lantern into bushes before you walk up to them (instead of facechecking or walking near them) to check for enemies.
Lantern also absorbs souls near it, so if the enemy is zoning you from your souls in the laning phase, you can use it to grab souls out of reach.
and of course, your hook reveals stealthed enemies, including stuff like vayne Q during her ult. basically, if an enemy tries to escape using stealth, you can rip them right out of it by hooking them (while they are invisible)
or 
is cause overall they are kinda OP at the moment (they are in every LCS game. I mean lets be honest,lulu can nasus somebody,slow by 80% at level 1 and make somebody worthless AND slowed...jayce bring damage that's kinda higher then it should be...)and I don't want to get a champ that may get nerfed in a 
fashion.
so enter 
. yeah. wonder how many people were offended by his passive name...im not but others may be xD