When the hotbed springs are under tension they tend to buckle, there are a few solutions out there so nothing really new here, but wanted to tweak something to my particular aims.
These firstly cup the springs helping to prevent buckling, I have placed them at the top and the bottom. The plastic interface has reduced a minor rattling noise from the bed I was experiencing, due to the metal of the spring on metal on the metal washer. When you fit the cups at the top and the bottom it also places the springs under greater tension, which creates a very ridged platform, there is a side effect to this!
I now needed to fit a new Z-axis limit switch bracket so I could tweak the height of the limit switch by about 1mm, this allowed the optimum height keeping the springs under fairly high tension. This one works very well and is nicely designed http://www.thingiverse.com/thing:906366
So if you have a 3mm glass plate, print the Z-axis adjuster first, assuming you don't have one already.
These are printed in PLA, as far as heat is concerned I placed these under the 3mmx8mm steel washer, I run my bed upto 60C and don't foresee any issues with even higher temperatures. These are placed at the corners of the heatbed and the thermistor is measuring the central temperature, the periphery of the heatbed will always be lower, as well as the 3mm screw attached to the lower plate assembly acting as a heatsink.