i use the insulated ones that zip all the way round like a suitcase. Once a week I put it completly opened in the top rack of the dishwasher and hang it out to dry after (I don't use 'dry' on the dishwasher)
I find they last the year.
Sandwiches go in a lock and lock plastic box and instruct all contents of lunchbox afterwards go in that when finished (especially yoghurt pots!) so not to muck up the lunch bag.