I try to use just use gesture + construction with people (though for difficult poses I'll compare my drawing to a photo or a magic poser model afterwards and correct) but a library of 3d objects to use as references for little everyday things/props would be really useful, cars, guns, etc., you can cheese some but not even most of them, pictures are 2d and finding new ones requires a internet connection or having access to the thing itself. There're so many things that you've seen hundreds of times but then when you try to draw em it's like, what does this look like again?