From my experience in ootheca collecting here in Colorado the best thing that you can do in this area is to plant some dense flower-producing shrub such as potentilla or wild rose. I think the reason why they congregate on these bushes is for a good supply of food, good ootheca laying area, and for a good area to hide. Of course, this is mainly for European mantids. I don't know much about collecting the ootheca of other varieties of mantids. I hope this helps.
-Jay