I have been uploading .dae (aka collada) into SL for a year and this is what I know of it from experience and from picking up tips in the forums and the in-game Builders groups (whose staff mostly specialise in pc's / prim / blender / gimp):
1st method: reduce complexity of original object, using built-in (Blender apparently has this) or bounce the model between eg; BalancerLite (is free and works on mac) and your 3D app (I use Cheetah3D for mac).
2nd method: lower the physics setting (high, low, extra low) for the upload in the SL viwers mesh upload screen. This renders the object in its full glory but without the avatar and the shape interacting as if the shape is solid, only some of it will be solid. This is not recomended for building sections eg; door frames.
3rd method: upload and set it to Convex Hull so it reads as 1 Prim no matter how complicated the shape is. You will need to set it to Phantom otherwise the hull will push avi's away from the object. If you need the object to have physics, Link it to an overlaid Prim which is set to Alpha texture (making it transparent which is not the same as Invisible texture, invisible texture can be still seen from some angles in some viewers because it has a dark patch on it).
4th method: link everything together to reduce Land Impact (does not reduce prim count). There are some restrictions on this (see the Convex Hull link above for a great article by Jeremy Linden detailing everything about this).
I picked Loki's link up from this thread, it has taught me a lot about LoD Level of Detail: check it out.