Garbage in, garbage out. If whatever it is you're using for input is functionally low resolution, so will be your output. Jacking up the print resolution on a crappy bitmap will do little to nothing to fix the problem. Likewise, increasing the resolution of a bitmap of dubious quality is equally futile. You need to do whatever you can to produce a high quality input file and not waste your time trying to find some magic collection of printer settings that will make everything just right. There aren't any.
You might have a go at running the input file through an AI engine such as Gigapixel, PhotoZoom, etc. and try to produce a 150ppi image where each pixel is not a part of a larger collective pixel or, more precisely, a large pseudo-pixel. Always keep in mind that in order to reduce a bitmap's size you have to discard information. In order to enlarge a bitmap you have to create new information. A decent AI engine does this sort of thing quite well. If you do manage to produce an acceptable bitmap then print it at 720dpi and all will be well. Printing at more than four times the input resolution doesn't hurt anything but is a waste of time. 4x plus or minus is the optimum ratio.