#!/bin/sh
while true
do
    echo "-------`date`--------"
    echo "tt%MEMt%CPU"
    ps ax -o comm,%mem,%cpu | sort -nk3 | tail -n 3
    sleep 30
done

$ ./monitor.sh
-------Mon Feb  4 20:00:51 CET 2013--------
                %MEM %CPU
java             9.1  3.6
Xorg             3.3  4.9
firefox          8.1 12.2
        ---
Xorg             3.3  4.9
firefox          8.1 12.2
java             9.1  3.6

chmod 744 monitor.sh
./monitor.sh > usage.log &