Add this to your .bashrc to create a backup of your bash history every time you open a new terminal, but not more than one time per hour(find -mmin +60). This will backup your bash history if it has more lines than the backup file.

backup_history.bash

export HISTSIZE=""

HIST_FILE=~/.bash_history
BACK_FILE=~/.bash_history_backup
if [ ! -f $BACK_FILE ];then touch -d "2 hours ago" $BACK_FILE;fi
if test $(find $BACK_FILE -mmin +60); then
        HIST_SIZE=$(cat $HIST_FILE|wc -l)
        BACK_SIZE=$(cat $BACK_FILE|wc -l)
        GROWTH=$(($HIST_SIZE - $BACK_SIZE))

        if [ $GROWTH -lt 0 ];then
                echo Looks like your bash history has problems...
                echo You can restore with cp $BACK_FILE $HIST_FILE
        else
                cp $HIST_FILE $BACK_FILE
        fi
fi