Awlgrip's non-skid additive is excellent - I prefer the coarse stuff. We brushed a deck using awlgrip and it looks better than our friend's professionally sprayed non-skid. Its also very easy and cheap to do a 'perfect' job.
1) Prep deck. We knocked down the old molded pattern and it looks very slick.
2) Paint deck with awlgrip high build primer (roller and brush work fine). Cure, sand lightly. This will give you a very flat surface.
3) Paint a coat of your deck color (again role and brush. Let dry, but re-coat before cure.
4) Paint another coat of your deck color, while its wet, knock a bunch of holes in the top of the paint can that the non-skid additive comes in (it becomes a big salt-shaker). Sprinkle the decks liberally. This additive is relatively cheap, so really cover it. Let this coat dry, not not cure. The additive will stick in the wet paint.
5) Gently brush and vacuum the excess non-skid. Only walk on it in socks so you don't scuff off the grit. Roll 2-3 more coats of deck color over the non-skid. The more coats you add the more durable it becomes, but your loose more of the aggressiveness of the grit. We did 2 coats and its been super tough and very very grippy. I'd do three next time, or at least assess at 2.
The key here is once you start laying on coats of the deck color, you must re-coat within 24 hours (check time and temp with instructions) so it doesn't cure. Its epoxy based, so once it cures you have to sand and clean if you want to recoat. Since we're talking non-skid here, sanding is not and option. Also, don't be tempted to apply heavy coats of awlgrip. This paint flows beautifully and makes the brush marks disappear. Because its flows readily, a heavy coat will sag. I'm typically a fan of spraying, but rolling and brushing works great on decks and shortens your cleanup time dramatically.
Also, don't use crushed walnuts or sand (popular answer to this question). I've heard of some folks doing this... A purpose made additive is cheap, inert and works great.