This brings us to the second tip, simplify the objects your joining. I’m not saying alter the object in any way,  just select the faces from both object that will be touching when you join them and detach them. Then do the boolean on the isolated parts and reattach the other objects to them, then weld the verts back together. This allows you not only to join more complex objects together, but also allows you to keep you object from having a ton of extra unneeded edges added to it like boolean normally dose. So I recommend you use this technique even when its not required to get the objects joined, as its often faster to do a detach, attach and weld than it is to delete a bunch of extra edges and clean up any other messes that boolean leaves.

Finally when you have objects so complex that even simplified they can not be booleaned, there is one last trick that at lest for me has never failed. But before you try it I suggest reevaluating how badly this object needs to be one, although I believe this last trick will work damn near 100% of the time, it can be rather time consuming and just might not be wroth the effort. You’ll want to still use the seconded technique with this even tho it is not always needed because it will save a lot of time in clean up. After detaching both of the objects to be joined duplicate each one. Now pick one object and boolean it with another, but instead of using union or subtraction or whatever your were going to instead select cut refine. Then preform the same operation on the second object by the first, you should now have two objects with more edges going through them where the other object intersected them, you may noticed a few edges seem to be missing, 3DSM will often add the verts but miss connecting a few edge. Simply go to vert mode and manually connected them. Now go delete all the overlapping faces so that you will be able to weld the objects together. Next join the objects together and weld the verts, you may have to add or remove some verts or use spot weld, it can be a bit tricky to get them joined as one object and looking good but have a little patience and you’ll get it. After that reattach the other parts and you should have one solid object.

A few other things to remember while working with booleans is never to move the object until the operation is complete, save before attempting a boolean as they may crash 3DSM, and save a backup before attempting more complex booleans as you will often reach the undo limit and may need to go back. Also clean up as you go, remember to go to border mode and check for edges after every boolean and clean them up. Leaving them there may make it harder to clean up later or impossible to preform more operation on the object without distortion. Remember these tips and soon the boolean operation will become one of your best friends instead of a dreaded last resort.