What I'm trying to make is a one-way piston.
When I start the level, it needs to be inactive, then a switch is pulled and it travels the full maxiumum distance and then stops permanently.
Perhaps I should exlplain what I'm trying to do, I just need a piston to launch the entrance over to the end and stay there after I enter the level.

I've tried various things but I can't get it to work correctly, It's probably an easy fix

If I remember correctly isn't it only to square click the switch, and change it so it only goes one way. Or something like that?

Was soo long since I played LBP :(

first create a piston and set the min/max length to whatever position you want the piston to travel to/from. As is the piston should move back and forth at whatever speed you set the piston to, keep it slow for the the moment.

next, create a two-way switch to connect to the piston and when selection the control type use "direction". when you pull the switch to one position the piston will go to it's min position and in the other direction it will go to it's max position.  speed up the piston some and you it will basically warp itself to the desired position (too slow and noobs will get themselves stuck in odd places in your level.  basically it has to move faster then someone can kill themselves.)

well this is off the top of my head and maybe too convoluted (but honestly kinda fun, I love making these type of switches), you can set the switch to dissolving material which will send a ball falling which has a proximity switch on it and will fall through the field of the switch which is connected to your piston. Time the fall/speed of the piston perfectly and it'll only go as far as you want it to and then stop.

edit: Kitler has found the right way to do it, but my way will look cooler.  You could even put a rocket in there somewhere.

You have to use a magnetic switch.

So have the piston linked to the magnetic switch located in another part of the map. Create a magnetic key above the magnetic switch and separate the two with a destroyable block. Then create button that the player can activate to destroy the wall.

So the magnetic switch won't activate, and the piston is at minimum length. The player pushes the button, the block is destroyed, the magnetic key falls and activates the magnetic switch. The piston extends to maximum length. As long as nothing happens to the magnetic key/switch, the piston will remain extended.

At least, that's how I think you do it. I don't own LBP anymore, and haven't played in a long time.

I forgot to specify that I was looking for an automatic way. Do you know how I could use a sensor switch or some other contraption to make it go on it's own without pulling a switch?

yeah, sure.

make it just like above except instead of a two-way switch use a magnetic switch.  make it so that when the key is away from the switch the piston is at the start of your level and when the key is near by the piston is at the end of your level. attach the switch and the key to two different blocks with the key on the block above the switch.  seperate the two blocks by a block of dissolve material so that when the disolve material disolves the key falls down and attivates the magnetic switch.

now to active the disolve material without pulling a switch....create a proximity switch near your entrance, the control type is "on/off".  when someone enters your level, the proximity switch is immediately activated, the dissolve material dissapears and then your entrance ends up on the other side of your level.

oh, and when testing your level make sure you aren't in edit mode cause otherwise you'll find out your switch works great only to have to rebuild it. xD

Tight, I got it thanks. I think I'm almost finished with the level.