I found it best to go down to the local shops for a long weight, then hang em on sky hooks for a few days. This stretching makes them more pliable, last longer and also less susceptible to the temperature differences Jedi mentions.
If you need extra weight, I find using a box of valve clearances is an ideal starting point, then just just add more valve clearances as required.
Just to be on the safe side, I used to hang them on rubber nails to minimise the risk of over stretching them.
If you are finding they still are not pliable enough, you could try pouring a couple of buckets of steam over them, as I found this also helped.
If you need extra weight, I find using a box of valve clearances is an ideal starting point, then just just add more valve clearances as required.
Just to be on the safe side, I used to hang them on rubber nails to minimise the risk of over stretching them.
If you are finding they still are not pliable enough, you could try pouring a couple of buckets of steam over them, as I found this also helped.