As xkaes points out, there are myriad approaches; loupes with and without opaque skirts and tilting capabilities, free-held magnifiers, clip-on loupes for your glasses, strong magnifying glasses, etc., etc. All of these will work; it boils down to personal preference and situation.
A couple of things to be aware of though: with short-focal-length lenses, it is often difficult to see into the corners of the image with a skirted loupe that doesn't tilt. Tilted loupes are nice, but pricey. Using a free-held magnifier solves the problem but introduces another; you have to move the magnifier back and forth to find the right distance from the frosted side of the ground glass for focusing. This gets pretty easy with practice, however, so don't shy away from this option. Personally, I use a 5x-6x free held magnifier.
Using high-powered magnifiers in the 8x-10x range seems to bother some, especially in conjunction with Fresnel screens. The Fresnel lines or a course grind on the ground glass can be distracting at high magnification (I've used 8x loupes with zero problems, even with Fresnel screens).
Some use 4-5 diopter reading glasses and have done with it, not needing more magnification for critical focus. I find that I need more magnification than that.
Another issue that doesn't get addressed often is general viewing of the ground glass for composing, etc. (i.e., not critical focusing. If you're far-sighted or suffering from presbyopia (which we all will at some point), finding a comfortable working distance from the ground glass can be a problem. This is where reading glasses come in handy for me. I use 3-diopter reading glasses to get me closer to the ground glass for general viewing; it helps me keep the dark cloth around my head better and lets me get close enough to see details. Still, for critical focusing, I use the 6x magnifier (with or without the reading glasses - it doesn't seem to make a difference).
Hope this helps,
Doremus