Though I haven't done a completely separated cake in years (and I do wish they'd come back in fashion), I did to a 4 tier a few weeks ago that had a separation for flowers between the 2nd and 3rd tiers.
I used the Coast Tier Separator System [ http://cakedeco.com/cgi-bin/webc.cgi/st_main.html?p_catid=266] and really liked it. There's a little center pin on the plate that you center and press into the bottom of your cake board before icing your cake that helps center and secure the cake on the plate. Super easy. I am pretty sure the SPS that people use here works much the same way.
Quote by @%username% on %date%
%body%