try to find items around the house to use in training. i use a box with phone books in it as a step sometimes. for a higher step i use a sturdy chair or my bed- about 19in high.
put a heavy backpack on. or if you have an infant put them in a carrier on you. try different items- liq. laundry det, 5 gal water jug, or just anything random in your house.
also experiment with different heights/speeds when you do the exercises. you can see an example of this in cathe friedrich's w/o. such as 'low ends' when you do a lower plie/lunge/squat and only slightly go up instead of all the way up. try looking at her Legs and Glutes w/o. this will also give alot of ideas you can try, or any of her w/o. you can watch clips on her website or on utube. if you decide to get some of her w/o, the down loads are very affordable. 3, 8, or 15 bucks each depending on what kind of w/o u want.
also about.com has a good chart of different moves beginner,inter, advanced. i have the one for lower body, i think theres one for upper body too.
just try to mix it up so your body's not too used to the moves.