I straighten 3mm and ribbed tubes and soft axles from display models by heating with a hairdryer [not a hot air gun] and putting into a mould made of lego, or putting an axle down the middle. Flat plates get heated the same way, and stuck under a thick steel plate I use as a heatsink when welding, but a shedload of books will do. For beams I heat them for longer and them pin them with non-slotted pins to a frame made of studded beams, and then squish the whole lot for a few hours. I think I've thrown away three beams out of 4K or so. It does take practice, but you can get most Lego surprisingly hot before it deforms irretrievably. The trick is slow gentle heat, hot air guns heat the surface too fast,