Extention, extention, extention.
If you've got your Uke extended with your elbow up under his how is he going anywhere?
then step through (omote) or tenkan (ura) and cut down, instant mat splat. beleive it or not shihonage is really hard to get out of when done properly.
as for Iriminage. OY! dont get me started. its so damn hard!
still i can do it but man its damn akward.