I was wondering if anyone has any tips on how to raise two cake tiers above the bottom cake tier. The cake is a 6', 8', and 10'. The reason for raising it is so between the 10' and 8' tiers roses can be added, so I don't need to raise it very high. I read somewhere about using styrofoam, but I don't exactly understand how this works. Thanks!

Yes, you can use styrofoam, but you would still need to use a support system underneath it.

Or just use the multipiece SPS legs to create the airspace and you'll lay the roses into that. You'd use the regular SPS legs for the rest of the cake.

Wilton push-in-pillars: (clear) - http://www.wilton.com/store/site/product.cfm?id=3E3119FF-475A-BAC0-56858282E1C02DE1&fid=63EB9DD6-475A-BAC0-5CC92A385169EC6C
(white): http://www.wilton.com/store/site/product.cfm?id=3E31469D-475A-BAC0-5B36A499A89FEA3B&fid=63EB9E14-475A-BAC0-520E6723DA1F2747
Used in this cake to elevate the top 2 tiers: http://cakecentral.com/gallery/1417802

Wilton Hidden Pillars: http://www.wilton.com/store/site/product.cfm?id=3E3119F0-475A-BAC0-5772682F766C019C&killnav=1
used in this cake to separate the tiers for floral insert: http://cakecentral.com/gallery/1303013

Thanks for all your tips and links!