Minecraft – Most efficient way to farm Slimes

put a slab on the square you are standing on and leave a 1 square gap in the direction the water is flowing from (use glass if you want to be able to see what's coming). The slab will prevent anything from being pushed through but you will automatically pick up the items as they approach.

Adding a picture to clarify:

Side view:

 G   W
UG   W
UG   W
SWWWWW

W = water
  = blank space
G = glass
U = where you stand
S = Slab

The chickens won't be able to move into where you are due to the half slab keeping them in the water tile beneath the glass.

Well in fact that's not so complicated when you think about it. Do you want to make it reversible ? You just have to create a clock with redstone. Do you want to do it in smp or single player mode ? Because I think in smp clocks are still buggy.
This is how you could do that :

       |----------[P] 
       |          [B] <- Block creation here ? (Such as cobblestone ?)
       |          [B]
       |          [B]
       |          [B] 
       |          [B]
       |          [B]
[Clock System]-[P][B][ ][ ][ ][ ][ ][ ][P] <- Optional 
[Ground]|---------[P] <- Optional                    

So here you'll have to put some switches nearly everywhere you need to (I'm lazy and that would be complicated to draw on this schematic). The left and upper pistons ([P]) are used to make those blocks horizontal, and the lower one and right one to make them back to vertical, so you'll have to : When you want to make them horizontal, activate the up and left pistons (connected to the clock), and vice versa when you want to make them vertical.
I may be wrong but that's how I would do that. Though maybe you should only put 6 blocks, because of the 7 block limit with pistons. Try and tell us if it works.

By the way if you don't want this to be inverted, just remove the right and lower pistons. That will only do vertical -> horizontal and not the reverse system.

If you told me exactly what you want to do with that I could help you a bit more than that. This technique is used a lot to make cobblestones farm. When you dig the last block of the horizontal line, another block is created and is pushed. Notice that this system will take some time and work block by block. I don't think there is a system that is faster and without using a clock (unless you want it to be manual)

Here is the link for the clock system : Clocks with repeater/pistons