In terms of prepping surfaces for floating patches you guys gotta try a can of spray adhesive. Just keep a can handy in your patch kit, a quick pass over the painted surfaces and then float over with your mud. Way less hassle than priming or sanding, sticks like :whistling, and you can coat right away
I'd never not tape any patch over 3-4" diameter and anything smaller I always cut the board at a 45 angle so the filler(synco confill's the best) has something to anchor onto.
You can get 12"-150' rolls of fiberglass mesh that are great for bigger patches as you can cut it to just the right size(check ebay).
One trick I've learned is to recess your tape/mesh by cutting back the paint/paper surrounding your patch using a utility knife so the patch sits flush with the existing wall - that way you don't need to float it out as much. And if your going to cut out a patch to run wires, fix plumbing, etc... - cut the patch out using your hole saw at a 45 degree angle all around. When you replace the patch it'll fit perfect and be recessed because of the material the saw removed. This in combo with above and it'll all end up sweet.
D'S