Shop early! Try buying things like bridesmaid dresses and shoes in the off-season or at an end of season clearance.
I knew last summer that I was getting married on the beach this summer. I knew I wanted my bridesmaids to wear embossed beach dresses from jcrew. So I waited until they went on sale and picked my color that way. I told the girls "You pick the style as long as it's this color and fabric." Most of them waited until final sale at jcrew went to an additional 30% discount and ended up with their dresses for under $40.
The scary thing about doing this is "What do I do if someone gains weight between now and then?!" Well the best suggestion I can give is -- order up one size bigger than what they wear today. This way, if they gain any weight, it will still fit. And if it's a little big at wedding time, it won't cost much to have it taken in at a tailor.
Another good tip for outfitting bridesmaids on a budget is to check out Saxon Shoes' end of summer sales. We ended up getting all the girls shoes for $25 a pair. And they are definitely a style shoe they can wear all summer long after the wedding.
Try and make sure everything you pick out is something your girls can wear again. No one wants to spend money on something they are only going to wear once.