Dealing with session expiry

I have some pages that use sessions to call out certain elements in the layouts. It allows user to decide how should their pages look for the duration of their visit. Unfortunately whenever a user leaves their browser idle past the expity time of the session on their next click pages fall apart and all styles etc. are lost. I understand that I can detect or at least anticipate expiry of the session and renew it before it does. The question is how? Thanks in advance R>