How do you guys bend pe parts so they don't twist. I am thinking when doing longer thinner parts that don't have a bend line.

I have used a straight edge (steel/aluminum) to make long bends on PE. Also a good set of smooth flat pliers can do the trick on smaller pieces.

Howdy godfather,

My first answer to you is to purchase a tool called a 'Hold 'n Fold'. It comes in a 4" version , as well as a larger 8" version. Basically a stainless steel clamping system that bends the PE part quite nicely I'm told. The only problem with the tool is, that it's hard to get a hold of.
Apparently the manufacturer can't keep up with the demand (good indication of the quality of the tool BTW).
Secondly, GSPatton's method mentioned above is a tried-and-true method that work's just great. The metal straight edged ruler is a method used by many successful modelers. Some on this very site.
Another method (my homegrown tool) is this 'contraption'. I took a small (the smallest available) QUICK-GRIP clamping tool manufactured by American Tool Companies, Inc.
You know, the black coloured, one-handed clamping tool normally used for carpentry with the yellow rubber clamping pads? Anyway, I carefully cut out a 2" x 1" square of stainless stock from one of my rulers, I then removed one of the little yellow pads from the jaws of one of the QUICK-GRIP's and mounted the stainless steel mini plate to the sliding jaw of the QUICK-GRIP. I have mounted (what was left of my ruler) onto a wooden base that is already clamped to the outside corner of my hobby bench. All I do when I want to bend PE is take my reconfigured QUICK-GRIP and clamp my PE onto the wooden base with the stainless base plate and proceed to bend away with my single edge razor blade!

Tread.

Oh, almost forgot. I also ground off approx. half the length of the sliding shaft so that my leg doesn't get caught on it beneath the table.

First, and before tryin' to do anything, I heat the PE parts red hot...It ' ll loose his springiness, and ' ll become softer...Then I use LEGO bricks: I saw off some of the round pieces to make room for the part, then I put another brick to sandwich the part...And hop, presto !
' ve a nice day...!

Seen a really cool sketch of an idea somewhere.
Its a hinge that can fold over completely. it has the same positioning of holes on both sides so a bolt at each end can be pushed through to fasten. Insert the PE to where you want it bended. Tighten the two bolts as tight as possible and using something flat and solid push over the PE to angle required. Sounds easy? and definetly a lot cheaper than the hold and fold.