Pruning advice, please

Hi - I have five established Ceanothus Arboreus Trewithen Blue which are doing really great but they're getting very congested up top and I'm a pruning novice and have no idea how much to cut them back once they're done flowering.  The branches are getting so long and heavy that they're tearing, or getting snapped on windy days.  They're about 3m high and have been pruned quite successfully to have clear trunks.  A couple are multi-stem, a couple not.  I'd be very grateful for any specific pruning advice, thanks!