To permanently increase the default cache storage on offline files go to Start then click on RUN and type gpedit.msc the group policy window will come up.
Drill down to Computer configuration\Administrative templates\network\offline files
Click on Enable and specify the percentage of the amount of the hard drive space you want use for offline files storage. For example, if you have a 80 GB hard drive, and want to use half of it (40Gb) enter 5000 (50.00%) on the default cache size. After you finish click on OK.
One of the things you might notice after you do this is, that the slide button to increase the amount of disk space for temporary offline file storage is greyed-out. That is-