Something

#!/bin/sh
set -x

BACKUP="/me/backups"
DIRS="/me/projects"
EXCLUDE="--exclude=*~"

DATE1=`date`
DATE2=`date +%Y%m%d_%H%M%S`
HOST=`uname -n`

FULL="0"

if [ -e $BACKUP/FULL ]; then
    FULL=`cat $BACKUP/FULL`
fi

if [ "$FULL" = "0" ]; then
    tar -cjpf $BACKUP/$HOST_$DATE2.tar.bz2 $DIRS $EXCLUDE 
    echo $DATE1 >> $BACKUP/FULL
else
    tar -cjpf $BACKUP/$HOST_$DATE2.tar.bz2 $DIRS $EXCLUDE --newer="$FULL"
fi

echo "Done"