Find folders > 100MB
du --all --one-file-system / | awk '{if($1 > 102400) print int($1/1024) "MB" " " $2 }'

List files, sort largest first (and tail it)
du -mxS / | sort -n | tail

Run apt-get clean to clear out apt's downloads

Disk Check Script
Looking for a script to check disk usage, and alert you when you go over a certain threshold?  Find one here:
http://proj.ri.mu/check-disk-space