I have been practicing bottling from the keg, but I'm not very good at it yet. I want to try to send some bottles to a competition just to see what the beer judges say.
I bottle from the keg from time to time, mostly to take somewhere, or to free up the keg when it is almost kicked. There is sticky on making a your own counter pressure bottle filler, but I am too lazy for that, so here is what I do.
First, I sanitize the tap and shove a Starsan soaked paper towel up the faucet and let it sit while I finishing prepping. Sanitize 6 bottles and let them rest upside down in a pot. Shut off the CO2 to the kegs I am not bottling from, dial down the pressure to 2-3 PSI and bleed the pressure from the keg that I am bottling from. Remove the starsan paper towel from the faucet and draw out about 3-4 ounces to clear the beer line (I find the first part of my pours to be lacking in carbonation). Then I stick a bottle onto the faucet, and pour the beer into the bottle - I tilt the bottle. It takes about 15 - 20 seconds to fill the bottle, and there should be very little foam. I leave about 1-1/2 inches of head space. I then shake the bottle side to side until a head starts to form and rises to the top of the bottle. Then I stick a sanitized cap on it and cap it. Set it off to the side and repeat.
I usually do this when I have 6 - 10 beers left in a keg, so I put them in the fridge. However, I have done this to carb and bottle a barleywine last year, and a year later they are fine. I think the biggest things are making sure the faucet is sanitized, and capping on the foam.
I am not saying this is a best practice, but so far it has worked for me.