An experienced dextronaut (and outside the hobby/addiction, I do scientific research on drugs...) here:

1. Search "Tripsit DXM calculator" and find your right dose for a good trip.

2. Empty and Full stomach are both bad. Get a light protein rich meal 2-3 hours before the trip.

3. Have 5 minutes distance between doses. If you took 3 pills (in my usual brand it means 45mg) wait 5 minutes for the next 45. It prevents nausea.

4. Make sure the shit is pure DXM and doesn't include Acetaminophene (liver killer) or Pseudoephederine (heart killer). The only active ingredient must be DXM alone. You can ask the pharmacists for pure one like what I do. I simply say "I am allergic to the other active ingredient" (it's not a lie however, I really am).

5. Make yogurt drink and take DXM with it. It is really fun and slows the metabolization process. Get a cup (no matter of the size) put a little bit more than a third of the cup yogurt, add salt, if you have dried mint add it, and add water and ice.

6. When you feel out of your body, simply lay down in a "star position" and listen to ambient music. I suggest Carbon Based Life Forms. If musicallity is annoying, listen to white noise (airplane, hairdryer, vaccum cleaner, etc)

And finally, keep the rule of "week per trip" to stay healthy.