The safest form of dxm is anything that lists dextromethorphan hydrobromide as its ONLY active ingredient. I always buy the 4 oz bottle of Robitussin Maximum Strength Cough. Not cough and cold, just cough.

Here's a link to some info about not using stuff like Coricidin cough and cold (btw the entire errowid site rocks, check out the other stuff they have on dxm):
http://www.erowid.org/chemicals/dxm/dxm_info2.shtml

Most ppl say to drink the whole thing, but I was pretty happy drinking just half (about 2 oz) the first time. But then again, I did have weed at the time, so I'm sure that helped a lot. Now I can take the entire 4 oz bottle, and that works out great.

Weed definitely has a huge synergistic effect when taken with dxm. If you toke when you're sorta peakin on dxm, it really makes the experience so much better.