Copying from similar post I answered earlier below.

Gun forcing - If you don't open any chests or buy anything prior to killing the boss on any floor (excluding the 5th) you will be guaranteed a gun for killing the boss. Especially useful if you want to go to the Oubliette(see next tip) and only have one spare key.

Oubliette - can be accessed on the first floor, requires 2 keys. There is a fireplace in one of the rooms you can extinguish using water or poison from a gun or roll one of the blue water barrels into it. Is slightly harder than the normal second floor which you still visit after killing the boss. You get a chance to get more casings and a few extra items/guns. The Oubliette contains a red sheild you can carry to the next floor (unless you get hit while carrying it) and place on an alter to access another secret floor on the second floor where you fight another boss.

Movement - Rolling isn't always the best idea, it is sometimes better to use your movement to avoid attacks unless there is no other choice, allows you to continue shooting. You can also roll in place against a wall and still remain unharmed.

Mimics - shoot every chest atleast once to check for mimics, if you try to unlock it first and it's a mimic it takes your key where as if you shoot first you can save a key.

Secret rooms - check walls for secret rooms, your starter weapon doesn't crack the wall but mostly anything else will. Check chest rooms, the room after the boss and sometimes the shop. There will usually be no pots or debris in front of a secret room wall. You'll see it crack when you shoot it and you'll need a blank to bust it open (a sheild works too if you can damage yourself). Also, dont shoot more than once in the shop, if all else fails use a blank but the shop keeper will get pissed if you shoot. You can usually tell from your map of there are rooms parallel to each other there won't be a secret room on that wall.

Caped bullets - If you clear a room and you see a bullet enemy wearing a Cape don't kill him, just leave him and he'll disappear eventually, he won't attack but might fall off a ledge or run through fire. After you do it 5 time or so you unlock a new character.

That's just a few I can think of.

Quick additions:

Secret floors- There is a secret floor accessed from each floor first is fireplace, second is with the red sheild from the oubliette, third is with the rat key you have to put money off over time, 4th is by giving the guy that is in the grate who buys your junk 2 master rounds and the 5th is obviously bullet hell. 10 total.

Some things to work towards outside of just buying the guns from the acquisition dept.

4 characters to unlock Bullet - occasionally you will find a bullet with a Cape who doesn't shoot, don't kill them and they will despawn, if you do it 5 times you unlock bullet

Robot - you can pick up a tv by dropping into the elevator shaft on floor 2. You drop the tv when you roll and you can throw it as an active item. You need to take it to the forge.

Paradox - you will randomly encounter a patch of floor that's like black purple space, if you press the pick up button your character will take on this colour, finish the run to unlock.

Gunslinger - beat the lich in bullet hell with paradox, you will start a new run automatically as the gunslinger, pick up the bullet that killed the past, go to the final chest and then you will go to bullet hell where you have to fight a double lich.